MNT: Fix CI test suite

Author: kernc

backtesting/test/__main__.py

@@ -1,6 +1,8 @@
+import sys
 import unittest
 
 suite = unittest.defaultTestLoader.discover('backtesting.test',
                                             pattern='_test*.py')
 if __name__ == '__main__':
-    unittest.TextTestRunner(verbosity=2).run(suite)
+    result = unittest.TextTestRunner(verbosity=2).run(suite)
+    sys.exit(not result.wasSuccessful())