Ignore TEST_TAG environment variable when not defined during testing

bindings/python/cntk/losses/tests/fmeasure_test.py

@@ -14,7 +14,7 @@ from _cntk_py import set_fixed_random_seed
 
 import pytest
 from os import environ
-@pytest.mark.skipif(environ['TEST_TAG'] in {'weekly', 'nightly'}, reason="Temporarily disabled this test in the Nightly/Weekly builds due to random failures.")
+@pytest.mark.skipif(environ.get('TEST_TAG') is not None and environ['TEST_TAG'] in {'weekly', 'nightly'}, reason="Temporarily disabled this test in the Nightly/Weekly builds due to random failures.")
 def test_fmeasure():
     a = np.array([[[[1., 1., 1., 0., 0.],
                     [1., 1., 1., 0., 0.],