dnd-character: add test for one-time ability init

[cmccandless]

I came across an implementation that simply randomized each property whenever it was called, something like:

class Character():
    def strength():
        return randint(3, 18)

These values should be calculated only once each. Checking one twice should verify that this is the case.

[rpottsoh]

LGTM.

[sshine]

Interesting!

And good catch!