test: the sigterm tests have some race conditions, let them re-run if needed

Author: nedbat

tests/test_concurrency.py

@@ -696,6 +696,7 @@ def random_load():                              # pragma: nested
 
 
 @pytest.mark.skipif(env.WINDOWS, reason="SIGTERM doesn't work the same on Windows")
+@flaky(max_runs=3)          # Sometimes a test fails due to inherent randomness. Try more times.
 class SigtermTest(CoverageTest):
     """Tests of our handling of SIGTERM."""