These tests take 20 seconds each to rebuild the entire world
in their respective modes. That's too much for short mode,
including all.bash on developer machines.

Keep doing it on builders and if someone runs 'go test' by hand
in that directory.

For #26473.

Change-Id: I3dc6955bc3aa7a20fd170efcde72a7d19b37bdbf
Reviewed-on: https://go-review.googlesource.com/c/go/+/177417
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>