regexp: skip long-running benchmarks if -short is specified

This CL helps race.bash finish in a reasonable amount of
time. Otherwise the Match/Hard1/32M benchmark takes over 1200 seconds
to finish on arm64, triggering a timeout.  With this change the regexp
benchmarks as a whole take only about a minute.

Change-Id: Ie2260ef9f5709e32a74bd76f135bc384b2d9853f
Reviewed-on: https://go-review.googlesource.com/c/go/+/201742


Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>