all: surround -test.run arguments with ^$

all: surround -test.run arguments with ^$

If the -test.run value is not surrounded by ^$ then any test that
matches the -test.run value will be run. This is normally not the
desired behavior, as it can lead to unexpected tests being run.

Change-Id: I3447aaebad5156bbef7f263cdb9f6b8c32331324
Reviewed-on: https://go-review.googlesource.com/c/go/+/651956
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>