nettest: log more details about err for checkForTimeoutError

For golang/go#52893

Change-Id: I2897c3f22b670af0aa2df1edbc9bbebdf1bafe59
GitHub-Last-Rev: bdccfc3
GitHub-Pull-Request: #132
Reviewed-on: https://go-review.googlesource.com/c/net/+/406435
Run-TryBot: Bryan Mills <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>

Author: ZekeLu

nettest/conntest.go

@@ -398,10 +398,10 @@ func checkForTimeoutError(t *testing.T, err error) {
 	t.Helper()
 	if nerr, ok := err.(net.Error); ok {
 		if !nerr.Timeout() {
-			t.Errorf("err.Timeout() = false, want true")
+			t.Errorf("got error: %v, want err.Timeout() = true", nerr)
 		}
 	} else {
-		t.Errorf("got %T, want net.Error", err)
+		t.Errorf("got %T: %v, want net.Error", err, err)
 	}
 }