internal/lsp/regtest: enable test for golang/go#37195

I believe that other changes in gopls/v0.4.0 also fixed the issue that
this regression test was written for. Re-enable it, and change it to
assert that diagnostics are cleared.

Fixes golang/go#37195

Change-Id: I4538186ad288d9c6f70cc450f948b62f3868941f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/228723
Run-TryBot: Rebecca Stambler <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: stamblerre

internal/lsp/regtest/diagnostics_test.go

@@ -51,13 +51,21 @@ go 1.12
 `
 
 func TestMissingImportDiagsClearOnFirstFile(t *testing.T) {
-	t.Skip("skipping due to golang.org/issues/37195")
 	t.Parallel()
 	runner.Run(t, onlyMod, func(t *testing.T, env *Env) {
-		env.CreateBuffer("main.go", "package main\n\nfunc m() {\nlog.Println()\n}")
+		env.CreateBuffer("main.go", `package main
+
+func m() {
+	log.Println()
+}
+`)
+		env.Await(
+			env.DiagnosticAtRegexp("main.go", "log"),
+		)
 		env.SaveBuffer("main.go")
-		// TODO: this shouldn't actually happen
-		env.Await(env.DiagnosticAtRegexp("main.go", "Println"))
+		env.Await(
+			EmptyDiagnostics("main.go"),
+		)
 	})
 }