ext: testlib loading tests from multiple directories

ext: testlib loading tests from multiple directories

We currently run regressions with the following command line

./main.py run [...] <directory>

Where <directory> is the positional argument pointing to the tests root
directory: Testlib will walk through the directory and load every
testsuite it encounters in its path.

./main.py run [...] <directory1> <directory2> ...

Allowing testlib to load tests from multiple directories will make it
possible to load out of tree regressions (living in an EXTRAS repository
for example)

JIRA: https://gem5.atlassian.net/browse/GEM5-905

Change-Id: I802d8753a18f4dfb00347252f031b5438e9be672
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40136
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>