cmd/go: avoid running slow tests on non-longtest builders

cmd/go: avoid running slow tests on non-longtest builders

Also annotate calls to tooSlow with specific reasons.

This will somewhat reduce test coverage on the 'darwin' builders until
we have darwin 'longtest' builders (#35678,#49055), but still seems
worthwhile to avoid alert fatigue from tests that really shouldn't be
running in the short configurations.

Fixes #58918.
Fixes #58919.

Change-Id: I0000f0084b262beeec3eca3e9b8a45d61fab4313
Reviewed-on: https://go-review.googlesource.com/c/go/+/474137
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>