Expand constexpr vector testing

[mnatsuhara]

GH-1407 implements constexpr std::vector (#45) along with basic test coverage for the various constexpr-ized vector functions. However, the test coverage regarding allocators is a little light. There is currently coverage that observes how many copies of the allocator are made, but not not testing non-equal allocators.

Once we finish up the initial implementations of all C++20 features, we should revisit #45 and improve test coverage here.