Add Tests for consistent formatting for MU indications

[plural]

+N[mu] should always turn into +N mu in stripped_text and does not need a period after it.

Add a test to enforce this.

[infinitenutshell]

Note that it's theoretically possible to have a +MU effect that does need a period, although I don't believe there is such an effect in the card pool currently. Namely, this would happen if the MU appears in a sentence instead of just being a standalone line e.g.

If [some condition], you get +1MU.

This may be a narrow enough futureproofing case that it's not worth working around until/unless it actually exists, but I'll let you make the call on that.