[basic] Expand FrozenMultiMap tests to also test SmallFrozenMultiMap in both small and small->large configurations.

I wrote this to do some basic expansion of the testing of FrozenMultiMap when I
was intending on making FrozenMultiMap support multiple keys. Upon reflection I
realized I did not actually need to do that... but rather than let these tests
go to waste, I decided to just upstream them.

Author: gottesmm