[3.13] gh-130917: update timer and workload in test_signal (GH-130918) (#130969)

The workload to advance the virtual timeout is too lightweight for some
platforms. As result the test goes in timeout as it never reaches the
end of the timer. By having a heavier workload, the virtual timer
advances rapidly and the SIGVTALRM is sent before the timeout.
(cherry picked from commit 78790811989ab47319e2ee725e0c435b3cdd21ab)

Co-authored-by: Diego Russo <diego.russo@arm.com>