test: run fake upstream initialization from the dispatcher thread (en…

…voyproxy#22856)

`//test/integration:tcp_proxy_odcds_integration_test` was observed to fail as follows:
```
==================== Test output for //test/integration:tcp_proxy_odcds_integration_test:
[==========] Running 24 tests from 1 test suite.
[----------] Global test environment set-up.
[----------] 24 tests from IpVersionsClientType/TcpProxyOdcdsIntegrationTest
[ RUN      ] IpVersionsClientType/TcpProxyOdcdsIntegrationTest.SingleTcpClient/0
[2022-08-25 20:22:46.750][3969][critical][assert] [test/integration/fake_upstream.cc:832] assert failure: !dispatcher_->isThreadSafe().
[2022-08-25 20:22:46.752][3969][critical][backtrace] [./source/server/backtrace.h:104] Caught Aborted, suspect faulting address 0x6b00000f81
[2022-08-25 20:22:46.752][3969][critical][backtrace] [./source/server/backtrace.h:91] Backtrace (use tools/stack_decode.py to get line numbers):
[2022-08-25 20:22:46.752][3969][critical][backtrace] [./source/server/backtrace.h:92] Envoy version: 0/1.24.0-dev/test/DEBUG/BoringSSL
[2022-08-25 20:22:46.773][3969][critical][backtrace] [./source/server/backtrace.h:96] #0: Envoy::SignalAction::sigHandler() [0x4c46a78]->[0x2cf2a78] external/com_google_googletest/googlemock/include/gmock/gmock-spec-builders.h:1267
[2022-08-25 20:22:46.773][3969][critical][backtrace] [./source/server/backtrace.h:96] #1: __restore_rt [0x7ffbdaa79420]->[0x7ffbd8b25420] ??:0
[2022-08-25 20:22:46.802][3969][critical][backtrace] [./source/server/backtrace.h:96] #2: Envoy::FakeUpstream::assertPendingConnectionsEmpty() [0x245bf0b]->[0x507f0b] ??:0
[2022-08-25 20:22:46.846][3969][critical][backtrace] [./source/server/backtrace.h:96] #3: Envoy::(anonymous namespace)::TcpProxyOdcdsIntegrationTest_SingleTcpClient_Test::TestBody() [0x1f596cb]->[0x56cb] ??:0
[2022-08-25 20:22:46.877][3969][critical][backtrace] [./source/server/backtrace.h:96] #4: testing::internal::HandleSehExceptionsInMethodIfSupported<>() [0x588b61b]->[0x393761b] ??:0
[2022-08-25 20:22:46.924][3969][critical][backtrace] [./source/server/backtrace.h:96] #5: testing::internal::HandleExceptionsInMethodIfSupported<>() [0x587c2bd]->[0x39282bd] ??:0
[2022-08-25 20:22:46.966][3969][critical][backtrace] [./source/server/backtrace.h:96] #6: testing::Test::Run() [0x5864ba3]->[0x3910ba3] /usr/lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:0
[2022-08-25 20:22:47.003][3969][critical][backtrace] [./source/server/backtrace.h:96] #7: testing::TestInfo::Run() [0x586576a]->[0x391176a] external/com_google_absl/absl/container/internal/raw_hash_set.h:1259
[2022-08-25 20:22:47.037][3969][critical][backtrace] [./source/server/backtrace.h:96] #8: testing::TestSuite::Run() [0x5865fbb]->[0x3911fbb] /usr/lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_algo.h:1925
[2022-08-25 20:22:47.089][3969][critical][backtrace] [./source/server/backtrace.h:96] #9: testing::internal::UnitTestImpl::RunAllTests() [0x5874a28]->[0x3920a28] envoy/registry/registry.h:509
[2022-08-25 20:22:47.114][3969][critical][backtrace] [./source/server/backtrace.h:96] #10: testing::internal::HandleSehExceptionsInMethodIfSupported<>() [0x588ddbb]->[0x3939dbb] envoy/registry/registry.h:0
[2022-08-25 20:22:47.160][3969][critical][backtrace] [./source/server/backtrace.h:96] #11: testing::internal::HandleExceptionsInMethodIfSupported<>() [0x587e683]->[0x392a683] /usr/lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/unique_ptr.h:154
[2022-08-25 20:22:47.191][3969][critical][backtrace] [./source/server/backtrace.h:96] #12: testing::UnitTest::Run() [0x5874568]->[0x3920568] envoy/registry/registry.h:508
[2022-08-25 20:22:47.237][3969][critical][backtrace] [./source/server/backtrace.h:96] #13: RUN_ALL_TESTS() [0x4878d51]->[0x2924d51] external/com_google_googletest/googlemock/include/gmock/gmock-spec-builders.h:1203
[2022-08-25 20:22:47.284][3969][critical][backtrace] [./source/server/backtrace.h:96] #14: Envoy::TestRunner::RunTests() [0x48783b1]->[0x29243b1] external/com_google_googletest/googlemock/include/gmock/gmock-actions.h:485
[2022-08-25 20:22:47.316][3969][critical][backtrace] [./source/server/backtrace.h:96] #15: main [0x4874c3a]->[0x2920c3a] external/com_google_googletest/googlemock/include/gmock/gmock-spec-builders.h:1181
[2022-08-25 20:22:47.316][3969][critical][backtrace] [./source/server/backtrace.h:96] #16: __libc_start_main [0x7ffbda897083]->[0x7ffbd8943083] ??:0
================================================================================
```

This is due to the race described by envoyproxy#22855. Making sure the dispatcher thread is running before starting the test avoids this problem.

Signed-off-by: Benjamin Peterson <benjamin@engflow.com>

test/integration/fake_upstream.cc

@@ -594,6 +594,11 @@ FakeUpstream::FakeUpstream(Network::DownstreamTransportSocketFactoryPtr&& transp
         ->mutable_max_rx_datagram_size()
         ->set_value(config.udp_fake_upstream_->max_rx_datagram_size_.value());
   }
+  dispatcher_->post([this]() -> void {
+    socket_factories_[0]->doFinalPreWorkerInit();
+    handler_->addListener(absl::nullopt, listener_, runtime_);
+    server_initialized_.setReady();
+  });
   thread_ = api_->threadFactory().createThread([this]() -> void { threadRoutine(); });
   server_initialized_.waitReady();
 }
@@ -644,9 +649,6 @@ void FakeUpstream::createUdpListenerFilterChain(Network::UdpListenerFilterManage
 }
 
 void FakeUpstream::threadRoutine() {
-  socket_factories_[0]->doFinalPreWorkerInit();
-  handler_->addListener(absl::nullopt, listener_, runtime_);
-  server_initialized_.setReady();
   dispatcher_->run(Event::Dispatcher::RunType::Block);
   handler_.reset();
   {