`<flat_map>`: Missing test coverage

[StephanTLavavej]

P0429R9_flat_map is missing some test coverage, especially for flat_map-specific members that aren't present for flat_multimap, although the latter is slightly affected.

• operator[] (extremely important, obviously)
• at() (both success and exceptional cases)
• emplace()
• try_emplace() (including hint and transparent forms)
• replace() (when successful, not just death)

In some cases there's a tiny bit of coverage but not focused testing.

Since we have decent libcxx coverage, I'm not too worried, but tests/std is what exercises our more exotic compiler options, and also EDG, so we should ideally have coverage there.