tests: Migrated 40.m5threads-test-atomic scons tests to testlib

tests: Migrated 40.m5threads-test-atomic scons tests to testlib

At present, the 40.m5threads-test-atomic tests fail as the SPARC binary
(generated from `tests/test-progs/pthread/src/test_atomic.cpp`) is not
present. This has been noted in:
https://gem5.atlassian.net/browse/GEM5-368

Change-Id: I7865826388be46cec06a201712081146a58518f2
Jira: https://gem5.atlassian.net/browse/GEM5-109
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25824
Reviewed-by: Ayaz Akram <yazakram@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>