Add async timer tests

sdk/test/event/async_timer/BUILD

@@ -0,0 +1,11 @@
+cc_test(
+    name = "dispatcher_test",
+    srcs = [
+        "dispatcher_test.cc",
+    ],
+    linkstatic = 1,
+    deps = [
+        "//sdk/src/event/async_timer:dispatcher",
+        "@com_google_googletest//:gtest_main",
+    ],
+)

sdk/test/event/async_timer/dispatcher_test.cc

@@ -0,0 +1,39 @@
+#include "src/event/async_timer/dispatcher.h"
+
+#include <chrono>
+
+#include <gtest/gtest.h>
+using opentelemetry::sdk::event::Timer;
+using opentelemetry::sdk::event::async_timer::Dispatcher;
+
+TEST(DispatcherTest, Timer)
+{
+  Dispatcher dispatcher;
+  std::chrono::steady_clock::time_point t1, t2;
+  auto timer = dispatcher.CreateTimer([&] { t2 = std::chrono::steady_clock::now(); });
+  timer->EnableTimer(std::chrono::milliseconds{100});
+  t1 = std::chrono::steady_clock::now();
+  dispatcher.Run();
+  auto duration = t2 - t1;
+  EXPECT_TRUE(duration > std::chrono::milliseconds{50});
+  EXPECT_TRUE(duration < std::chrono::milliseconds{200});
+}
+
+TEST(DispatcherTest, TimerOrder)
+{
+  Dispatcher dispatcher;
+  std::vector<int> order;
+
+  auto t1 = dispatcher.CreateTimer([&] { order.push_back(1); });
+  t1->EnableTimer(std::chrono::milliseconds{5});
+
+  auto t2 = dispatcher.CreateTimer([&] { order.push_back(2); });
+  t2->EnableTimer(std::chrono::milliseconds{2});
+
+  auto t3 = dispatcher.CreateTimer([&] { order.push_back(3); });
+  t3->EnableTimer(std::chrono::milliseconds{7});
+
+  dispatcher.Run();
+
+  EXPECT_EQ(order, (std::vector<int>{2, 1, 3}));
+}