[libcxx] [test] Fix the new alloc tests for Windows

Use ASSERT_WITH_OPERATOR_NEW_FALLBACKS where relevant to waive
the known cases where operator new isn't overridden as expected,
in MinGW DLL configurations.

Clarify the reason for why the fallback in
new.delete.array/new.size_align_nothrow.replace.indirect doesn't
work as expected, which can be considered a vcruntime bug.

Differential Revision: https://reviews.llvm.org/D151304