[libcxx][test] Split off debug mode tests

This continues the work started by @ldionne in 2908eb20.

The debug mode tests from

- libcxx/containers/sequences/vector/
- libcxx/strings/basic.string/string.access/
- libcxx/strings/basic.string/string.iterators/

similarly contain two tests in every file making the second test never
run. The patch splits the tests into separate files.

Reviewed By: Quuxplusone, ldionne

Differential Revision: https://reviews.llvm.org/D100592