pytest: Remove valgrind opt-out via tests marker

Reviewing ElementsProject#4391 I noticed that we have a sneaky way of opting out of
valgrind using the `slow_test` marker. I consider this dangerous as it
causes a false sense of security ("this test is being run under
valgrind" when really it isn't), as exemplified by some of the tests
even branching on whether we run under valgrind or not.

This is my attempt of making tests explicitly opt out of valgrind
testing via the `@unittest.skipIf` decorator, which is more honest, as
it doesn't run the test in a tweaked configuration, and reports a
skipped test. The tests are all already being run on CI without
valgrind, so we don't lose any coverage by this, but are more
respectful of the resources we have on CI.

tests/test_connection.py

@@ -166,7 +166,6 @@ def test_bad_opening(node_factory):
 
 @unittest.skipIf(not DEVELOPER, "gossip without DEVELOPER=1 is slow")
 @unittest.skipIf(TEST_NETWORK != 'regtest', "Fee computation and limits are network specific")
-@pytest.mark.slow_test
 def test_opening_tiny_channel(node_factory):
     # Test custom min-capacity-sat parameters
     #
@@ -2586,16 +2585,10 @@ def test_fulfill_incoming_first(node_factory, bitcoind):
 
 
 @unittest.skipIf(not DEVELOPER, "gossip without DEVELOPER=1 is slow")
-@pytest.mark.slow_test
+@unittest.skipIf(VALGRIND, "Too memory hungry under valgrind")
 def test_restart_many_payments(node_factory, bitcoind):
     l1 = node_factory.get_node(may_reconnect=True)
-
-    # On my laptop, these take 89 seconds and 12 seconds
-    if node_factory.valgrind:
-        num = 2
-    else:
-        num = 5
-
+    num = 5
     nodes = node_factory.get_nodes(num * 2, opts={'may_reconnect': True})
     innodes = nodes[:num]
     outnodes = nodes[num:]
@@ -2883,13 +2876,11 @@ def test_feerate_stress(node_factory, executor):
 
 
 @unittest.skipIf(not DEVELOPER, "need dev_disconnect")
-@pytest.mark.slow_test
+@unittest.skipIf(VALGRIND, "Reconnection slow due to process spawning")
 def test_pay_disconnect_stress(node_factory, executor):
     """Expose race in htlc restoration in channeld: 50% chance of failure"""
-    if node_factory.valgrind:
-        NUM_RUNS = 2
-    else:
-        NUM_RUNS = 5
+    NUM_RUNS = 5
+
     for i in range(NUM_RUNS):
         l1, l2 = node_factory.line_graph(2, opts=[{'may_reconnect': True},
                                                   {'may_reconnect': True,