[libc++] Refactor string unit tests to ease addition of new allocators

While doing this, I also found a few tests that were either clearly
incorrect (e.g. testing the wrong function) or that lacked basic test
coverage like testing std::string itself (e.g. the test was only checking
std::basic_string with a custom allocator). In these cases, I did a few
conservative drive-by changes.

Differential Revision: https://reviews.llvm.org/D140550


Co-authored-by: Brendan Emery <brendan.emery@esrlabs.com>