Fail if number of excuted tests != number of dicovered tests

this will report failing status in CI in case we have the
'Ran 0 tests in 46.486s' scenario.

Note: the check is placed after runner.run() to present the
message shown above which should make it more clear what went on.

Related: rhbz#1698366

0 run

test/composertest.py

@@ -132,4 +132,8 @@ def main():
     runner = unittest.TextTestRunner(verbosity=2, failfast=args.sit)
     result = runner.run(tests)
 
+    if tests.countTestCases() != result.testsRun:
+        print("Error: unexpected number of tests were run", file=sys.stderr)
+        sys.exit(1)
+
     sys.exit(not result.wasSuccessful())