go/packages/packagestest: set Config.Logf if the test is run verbosely

The Config.Logf field enables debug logging in go/packages, which is
exactly what we want when running a test in verbose mode to debug it.

For golang/go#60650.

Change-Id: I36b47e214860b5aec7c66042fc0ceb50c7062f1a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/502175
Reviewed-by: Michael Matloob <[email protected]>
TryBot-Bypass: Bryan Mills <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
gopls-CI: kokoro <[email protected]>

Author: Bryan C. Mills

go/packages/packagestest/export.go

@@ -217,6 +217,9 @@ func Export(t testing.TB, exporter Exporter, modules []Module) *Exported {
 		written:       map[string]map[string]string{},
 		ExpectFileSet: token.NewFileSet(),
 	}
+	if testing.Verbose() {
+		exported.Config.Logf = t.Logf
+	}
 	defer func() {
 		if t.Failed() || t.Skipped() {
 			exported.Cleanup()