[libcxx] [test] Change the generic_string_alloc test to test conversions to all char types

On windows, the native path char type is wchar_t - therefore, this test
didn't actually do the conversion that the test was supposed to exercise.

The charset conversions on windows do cause extra allocations outside of
the provided allocator though, so that bit of the test has to be waived
now that the test actually does something. (Other tests have similar
TEST_NOT_WIN32() for allocation checks for charset conversions.)

Also fix a typo, and amend the path.native.obs/string_alloc test to
test char8_t, too.

Differential Revision: https://reviews.llvm.org/D102360