<flat_meow>: Add tests for all flat_map constructors, deduction guides and then some

[vmichal]

Changed functionality:

• Fix optimization of heterogeneous flat_map::erase(OtherKey&&) when OtherKey is not key_type .

Tests:

• Test that flat_meow::insert, emplace_hint and similar functions accepting hint correctly handle correct hint (insertion takes place as close as possible to given hint)
• Test all (I believe) constructors and deduction guides of flat_map
• Add death tests for all _STL_ASSERTs in both <flat_map> and <flat_set> - unsorted/duplicate elements in ranges and size mismatch of key and mapped container.
• Test all heterogeneous lookup and erase operations (related to previous work in <flat_meow>: Don't do extra work in count(), contains(), and equal_range() #5994, see discussion in <flat_map>: count(), contains(), and equal_range() are doing too much work #5992 ).
• Extract common tests of container requirements and invariants from flat_set test and also apply them to flat_map objects.

Progress towards https://discord.com/channels/737189251069771789/1120873380615295027/1450952672982204468

Also to exercise every deduction guide. [...] I want to make sure that every way we construct the container adaptors, we establish the invariants (so we actually-sort and actually-unique input when necessary).