Add Catch2 test framework. Add example test. Add tests build instructions.

Tests/Example_test.cpp

@@ -0,0 +1,12 @@
+#include "catch.hpp"
+
+unsigned int Factorial( unsigned int number ) {
+    return number <= 1 ? number : Factorial(number-1)*number;
+}
+
+TEST_CASE( "Factorials are computed", "[factorial]" ) {
+    REQUIRE( Factorial(1) == 1 );
+    REQUIRE( Factorial(2) == 2 );
+    REQUIRE( Factorial(3) == 6 );
+    REQUIRE( Factorial(10) == 3628800 );
+}