Added new test: gcd2, but yet disabled

The gcd2 challenges requires the user to accept two arguments as
numbers and output the greatest common divisor of them. This is
to let the user have an exercsise to practice the Euclidian
algorithm.

The test is not yet enabled. It will be enabled in a later commit
to make it easier to rollback in case of problems.
Officially enable the gcd2 test

The gcd2 test was written in a previous commit but not enabled to
make it easier to roll it back in case of problems.

This commits officially enables the challenge.