Make test_ssti and test_xpath runnable standalone (#6136)

Every tests/test_*.py file provides an

    if __name__ == "__main__":
        unittest.main(...)

entrypoint so it can be run directly, except test_ssti.py and
test_xpath.py. Running either of those directly produced no output and
executed none of its tests; they only ran under `unittest discover`.

Add the missing entrypoint to both so they behave like the rest of the
suite. No test logic changed (62 and 48 tests respectively still pass).

tests/test_ssti.py

@@ -827,3 +827,7 @@ class TestStruts2Header(unittest.TestCase):
         # so no start/end marker is found -> no fabricated 'output'
         ssti._s2045Send = lambda url, action: "reflected: %s" % action
         self.assertIsNone(ssti._executeStruts2Header("http://target", "id"))
+
+
+if __name__ == "__main__":
+    unittest.main(verbosity=2)

tests/test_xpath.py

@@ -557,3 +557,7 @@ class TestRealXPathSyntax(unittest.TestCase):
                 self.fail("Boundary '%s' in '%s' with orig='%s' invalid: %s\n  payload: %s" % (bk, tkey, original, e, payload))
             self.assertIsInstance(count, int,
                 "Boundary '%s' in '%s' produced no count" % (bk, tkey))
+
+
+if __name__ == "__main__":
+    unittest.main(verbosity=2)