Allow flag to appear anywhere in arguments list in flags test

The flag passed in the `flags` parameter of `//test:flags_test` isn't
always guaranteed to be the first argument in the interpreter's
arguments list - other factors, e.g. overriding the value of the
plugin's `InterpreterOptions` configuration option when running the
test, can affect the argument order. Relax the assertion in the test so
it passes if the flag appears anywhere in the arguments list.

Author: chrisnovakovic

test/flags_test.py

@@ -7,4 +7,4 @@
 class FlagsTest(unittest.TestCase):
 
     def test_flags(self):
-        self.assertEqual('--test_flag', sys.argv[1])
+        self.assertIn("--test_flag", sys.argv)