I marked every test that takes more than 0.5 seconds on my machine
as something to run only when not in -short mode, or in -short mode
on the beefy linux/amd64, windows/amd64, and darwin/amd64 builders.
I also shortened a few needlessly-expensive tests where possible.
Cuts the time for go test -short cmd/go from 45s to 15s on my machine.
Should help even more on some of our builders and slower user machines.
Fixes #23287.
Change-Id: I0e36003ef947b0ebe4224a1373731f9fa9216843
Reviewed-on: https://go-review.googlesource.com/86252
Run-TryBot: Russ Cox <rsc@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>