cmd/go: add -benchtime to cacheable test flags

Add -benchtime to the list of flags that allow caching test results.

If -benchtime is set without -bench, no benchmarks are run. The cache
does not need to be invalidated in this case.

If -benchtime is set with -bench, benchmarks are run. The cache is
invalidated due to the -bench flag in this case.

Fixes #44555

Change-Id: I2eb5c9f389a587d150fb984590d145251d0fa2dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/304689


Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>