testing: use strings.LastIndexAny in (*common).decorate

testing: use strings.LastIndexAny in (*common).decorate

Change-Id: I14c521f4e599e14c71a52342b0ead5d0d4007f1e
Reviewed-on: https://go-review.googlesource.com/c/go/+/527655
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Kirill Kolyshkin <kolyshkin@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>