all: use ^$ instead of XXXX, NoSuchTestExists to match no tests

all: use ^$ instead of XXXX, NoSuchTestExists to match no tests

It's shorter and can't accidentally match unlikely test names.

Change-Id: I96dd9da018cad1acf604f266819470278f54c128
Reviewed-on: https://go-review.googlesource.com/c/go/+/524949
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>