Fix 2 more tests that fail sometimes in CI (#751)

openmmtools/tests/test_sampling.py

@@ -316,6 +316,7 @@ def test_with_unsampled_states(self):
     # on windows we get a ZeroDivisionError: float division by zero
     # when measuring the timing data
     @pytest.mark.skipif(sys.platform.startswith("win"), reason="Test fails on windows")
+    @pytest.mark.flaky(reruns=3)
     def test_without_unsampled_states(self):
         """Test multistate sampler on a harmonic oscillator without unsampled endstates"""
         self.run(include_unsampled_states=False)