tstest: do not panic when a panic logs

The panicLogWriter is too strict, and any panics that occur
get wrapped up in quotes. This makes it so that it will allow
panics to continue writing to Stderr without going through
logger.Logf.

Updates #cleanup

Signed-off-by: Maisem Ali <maisem@tailscale.com>

tstest/log.go

@@ -42,7 +42,7 @@ func (panicLogWriter) Write(b []byte) (int, error) {
 	// interfaces.GetState & tshttpproxy code to allow pushing
 	// down a Logger yet. TODO(bradfitz): do that refactoring once
 	// 1.2.0 is out.
-	if bytes.Contains(b, []byte("tshttpproxy: ")) {
+	if bytes.Contains(b, []byte("tshttpproxy: ")) || bytes.Contains(b, []byte("runtime/panic.go:")) {
 		os.Stderr.Write(b)
 		return len(b), nil
 	}