[libc++] [test] Constexpr-ify a couple of insert-iterator tests.

This should have been done in D96385; thanks ldionne for the catch!
Also, make the back/front inserter behavior tests a little more thorough,
which incidentally caught a cut-and-paste-bug in `nasty_list`, so fix that.

Differential Revision: https://reviews.llvm.org/D103318