Test for Ability Score Generators returning values above 18

[fluffymuffin27]

Currently we test for scores less than 0, but not for scores higher than 18, as noted on the wiki planning document.

[IMPrager]

Keep in mind that Point Buy changes things up a bit:

• Rather than testing for scores less than 0, you need to prevent users from lowering scores below 7.

Users still can't increase scores above 18 as far as I know.

[hamiltonium]

I've altered the test to raise an error when dexterity is greater than 18. For some reason, it looks like the coverage dropped a bit when I added that line of code.

[fluffymuffin27]

Coverage may have dropped because it might still consider whether the test files themselves are 'covered'. Not unusual.

Fixed by #4