add testcases for power method

test/test.cpp

@@ -5,4 +5,6 @@
 TEST_CASE( "some example calculations", "[calc]") {
   REQUIRE( evaluate("11+") == 2 );
   REQUIRE( evaluate("12+2-3*") == 3 );
+  REQUIRE( evaluate("22^") == 4 );
+  REQUIRE( evaluate("43^") == 64 );
 }