Replace custom assertContains() with unitest2 assertIn()

tests/first_stage_test.py

@@ -36,7 +36,7 @@ class CommandLineTest(testlib.RouterMixin, testlib.TestCase):
         stdout, stderr = proc.communicate()
         self.assertEquals(0, proc.returncode)
         self.assertEquals("EC0\n", stdout)
-        self.assertContains("EOFError", stderr)
+        self.assertIn("EOFError", stderr)
 
 
 if __name__ == '__main__':

tests/testlib.py

@@ -126,9 +126,6 @@ class TestCase(unittest.TestCase):
             assert 0, '%r raised %r, not %r' % (func, e, exc)
         assert 0, '%r did not raise %r' % (func, exc)
 
-    def assertContains(self, needle, hay):
-        assert needle in hay, "%r not found in %r" % (needle, hay)
-
 
 class DockerizedSshDaemon(object):
     def __init__(self):