misc/reboot: skip TestRepeatBootstrap on short builders

This test is slow and resource-intensive, and will rarely catch
failures. It is important to run sometimes, but probably a waste of
time on smaller (and especially reverse) builders.
Rather than hard-coding a list of small builders, only run it on the
longtest builders.

Fixes #35233
Fixes #30892
Fixes #49753

Change-Id: I25a9702e1f541246ea200fd7c79414ca5f69edae
Reviewed-on: https://go-review.googlesource.com/c/go/+/366538


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>