util: Teach the m5 utility's scons how to run unit tests.

util: Teach the m5 utility's scons how to run unit tests.

This may be directly in the case of native tests, or through a user
level QEMU binary for non-native tests. scons is smart enough to expect
to be able to run native tests always, and non-native tests only if a
qemu binary has been found.

To tell scons to run tests in a particular category, you can use a
command of this form:

scons build/[category]/test/

where category is either an "abi" like sparc or x86, or "native" for
tests which don't do anything target specific and so can be run on the
host.

There will be two directories under .../tests, "bin" and "result". "bin"
is where the test binaries themselves will be built, and "result" is for
the results of running those binaries.

Change-Id: I6450ab4a97169f8a01292d946bfac18008b0430c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27752
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>