Adding chapter 5 source code

Chapter5/myunittest/tests/tests_mycalc/test_mycalc_subtract.py

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+#test_mycalc_subtract.py test suite for substract class method
+import unittest
+from myunittest.src.mycalc.mycalc import MyCalc
+
+class MyCalcSubtractTestSuite(unittest.TestCase):
+    def setUp(self):
+        self.calc = MyCalc()
+
+    def test_subtract(self):
+        """ test case to validate two positive numbers"""
+        self.assertEqual(5, self.calc.subtract(10 , 5), "should be 5")
+
+if __name__ == '__main__':
+    unittest.main()
+