gopls/internal/test: await quiescence in TestInconsistentVendoring

TestInconsistentVendoring was awaiting only the initial workspace load
before capturing diagnostics and running their code action. But opening
a/a1.go also triggers diagnostics. This can cause the file lock failures
observed in golang/go#64229. To fix this, wait for the change (the
didOpen) to be fully processed.

Fixes golang/go#64229

Change-Id: I747ea22db02b8def4bf7764714db7f901891792d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/558076
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Findley <rfindley@google.com>

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

@@ -51,8 +51,7 @@ func _() {
 	).Run(t, pkgThatUsesVendoring, func(t *testing.T, env *Env) {
 		env.OpenFile("a/a1.go")
 		d := &protocol.PublishDiagnosticsParams{}
-		env.OnceMet(
-			InitialWorkspaceLoad,
+		env.AfterChange(
 			Diagnostics(env.AtRegexp("go.mod", "module mod.com"), WithMessage("Inconsistent vendoring")),
 			ReadDiagnostics("go.mod", d),
 		)