[libc++] Overhaul all tests for assertions and debug mode

Prior to this patch, there was no distinction between tests that check
basic assertions and tests that check full-fledged iterator debugging
assertions. Both were disabled when support for the debug mode is not
provided in the dylib, which is stronger than it needs to be.

Furthermore, all of the tests using "debug_macros.h" that contain more
than one assertion in them were broken -- any code after the first
assertion would never be executed.

This patch refactors all of our assertion-related tests to:
1. Be enabled whenever they can, i.e. basic assertions tests are run
   even when the debug mode is disabled.
2. Use the superior `check_assertion.h` (previously `debug_mode_helper.h`)
   instead of `debug_macros.h`, which allows multiple assertions in the
   same program.
3. Coalesce some tests into the same file to make them more readable.
4. Use consistent naming for test files -- no more db{1,2,3,...,10} tests.

This is a large but mostly mechanical patch.

Differential Revision: https://reviews.llvm.org/D121462