Add a test driver for functions module.

test_functions.cpp

+// test_functions.cpp
+//
+// The Makefile expects that the project solution has main() in a file
+// named project*.cpp.  For your own testing purposes, .cpp files that
+// begin with 'test' can include a main(), that will not conflict with
+// the project solution main().  Also, the test*.cpp files will not be
+// included in any submission archives.
+//
+// Tests can be built in two ways:
+//  $ make test_functions  // Make the test in test_functions.cpp with all
+//                         // other source (and header) files.
+//  $ make alltests        // Make all tests in test*.cpp with all other
+//                         // source (and header) files.
+
+#include "functions.h"
+
+int main () {
+    calculate(2, 2);
+    calculate(11, 31);
+
+    return 0;
+}