testing(bigquery/storage/managedwriter): fix go-cmp diff usage (#11459)

This switches usage of sync.Mutex in options testing from allowing
unexported to ignoring outright.  There's chances to sync in the latest
versions of go head that aren't compatible with AllowUnexported, and
the comparison isn't relevant to the code under test.

bigquery/storage/managedwriter/options_test.go

@@ -342,7 +342,7 @@ func TestWriterOptions(t *testing.T) {
 
 		if diff := cmp.Diff(got, tc.want,
 			cmp.AllowUnexported(ManagedStream{}, streamSettings{}),
-			cmp.AllowUnexported(sync.Mutex{}),
+			cmpopts.IgnoreTypes(sync.Mutex{}),
 			cmp.AllowUnexported(versionedTemplate{}),
 			cmpopts.IgnoreFields(versionedTemplate{}, "versionTime", "hashVal"),
 			protocmp.Transform(), // versionedTemplate embeds proto messages.