[libc++] Complete refactor of tests for operator new

I stumbled upon the `operator new` and `operator new[]` tests while
investigating an issue with `operator new` when exceptions are disabled,
and I realized that our test coverage was incomplete. This patch refactors
all the `operator new` and `operator new[]` tests to add consistency and
better coverage for scenarios in which it should be possible to override
an operator indirectly by defining another one (for example new(size_t, nothrow)
should use new(size_t) if it has been provided).

This is intended to be a NFC setting up the terrain for some refactoring
work and bug fix in operator new.

Differential Revision: https://reviews.llvm.org/D150408