CI test 'test_toy_lock_cleanup_signals' occassionally fails

[casparvl]

Ran into an issue where the test_toy_lock_cleanup_signals test in CI failed for no apparent reason. @boegel confirmed this was a flaky test that fails occasionally and asked me to open an issue on it.

I don't have any further info on it, but see here for an example.

[boegel]

Output of failing test:

FAIL: test_toy_lock_cleanup_signals (test.framework.toy_build.ToyBuildTest)
Test cleanup of locks after EasyBuild session gets a cancellation signal.
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Traceback (most recent call last):
  File "/tmp/runner/a1a9cf08297f1c3dc8551fe19465b35dbcecf7ed/lib/python3.5/site-packages/test/framework/toy_build.py", line 3536, in test_toy_lock_cleanup_signals
    self.assertTrue(regex.search(stderr), "Pattern '%s' found in: %s" % (regex.pattern, stderr))
AssertionError: None is not true : Pattern '^WARNING: signal received \(3\), cleaning up locks \(.*software_toy_0.0\)\.\.\.' found in: WARNING: signal received (3), cleaning up locks ()...