misc: Suggest usage of ALL instead of NULL in TESTING.md

We can now reallty build *all* unit tests using ALL.  The issue with
NULL is that some ISA specific unit tests were not actually run
therefore the user was not really running all available unittests

Change-Id: I4aab8e9a093d1f6ef4d473b0dbe240bf72dc739b
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/64311
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
1 file changed