gopls/internal/test/integration: fix race

Gotta install the message collector before we trigger
the thing that causes the message to be sent!

Fixes golang/go#74072

Change-Id: Ia110703ff0a7ad6b32939be9cd6615cef649e6bf
Reviewed-on: https://go-review.googlesource.com/c/tools/+/680756
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>

Author: adonovan

gopls/internal/test/integration/env.go

@@ -202,6 +202,10 @@ func (a *Awaiter) onShowMessage(_ context.Context, params *protocol.ShowMessageP
 // notifications. Call the resulting func to deregister the listener and
 // receive all notifications that have occurred since the listener was
 // registered.
+//
+// ListenToShownMessages should be called before the operation that
+// generates the showMessage event to ensure that the event is
+// reliably collected.
 func (a *Awaiter) ListenToShownMessages() func() []*protocol.ShowMessageParams {
 	id := nextAwaiterRegistration.Add(1)
 

gopls/internal/test/integration/misc/generate_test.go

@@ -119,8 +119,8 @@ var x = 1
 `
 	Run(t, src, func(t *testing.T, env *Env) {
 		env.OpenFile("a/a.go")
-		env.RegexpReplace("a/a.go", "var", "const")
 		collectMessages := env.Awaiter.ListenToShownMessages()
+		env.RegexpReplace("a/a.go", "var", "const")
 		env.Await(env.DoneWithChange())
 		messages := collectMessages()