added test for extraction of a negative constant without math operators

src/test/py/test_read_math_function.py

@@ -40,6 +40,8 @@ class read_function(unittest.TestCase):
         self.assertEqual(simplify_easy_math_function("v", "42^(2)+1-3*4/3"), 1761)
     def test_extraction_linear_function_constant_without_math_operators_addition(self):
         self.assertEqual(extract_constant("42x+5"), ["5", "42x"])
+    def test_extraction_linear_function_constant_without_math_operators_subtraction(self):
+        self.assertEqual(extract_constant("42x-5"), ["-5", "42x"])
 
 if __name__ == '__main__':
     unittest.main()