tests, misc: Updating TESTING.md to describe running unit tests

tests, misc: Updating TESTING.md to describe running unit tests

Previously we did not have any documentation discussing how gem5's GTests
were built and executed. TESTING.md has thereby been updated to highlight
how this is done. These unit tests should be run prior to each new
submission to Gerrit. This has been noted in TESTING.md also.

Change-Id: If5867fa0a2e4f6ea0921191a51e779c19a28117a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21479
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>