util: Make running m5 util unit tests opt-in.

util: Make running m5 util unit tests opt-in.

These tests don't run reliably right now for a few reasons, including
problems with QEMU, and apparently inaccurate information from g++-s
--print-sysroot option.

This may be revisited in the future if those problems can be sorted out.
For now, avoid tripping up new people who won't know to (or how to) work
around those sorts of errors.

Change-Id: Ide42e6c6b27159ff146b8495ae568d1fd377f4f4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28179
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>