cmd/go/internal/{clean,test}: lock testexpire.txt

Also check to make sure we don't overwrite a newer timestamp with an
older one.

testexpire.txt may be written concurrently, and a partially-written
timestamp may appear much older than the actual intended one. We don't
want to re-run tests that should still be cached.

Updates #26794

Change-Id: If56348e799f0e7be3c5bc91b4a336e23ad99f791
Reviewed-on: https://go-review.googlesource.com/c/146379


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>