[libc][NFC] add macro for fuchsia to switch test backend to zxtest

This moves utils/UnitTest/Test.[h/cpp] to LibcTest.[h/cpp] and adds a
new Test.h that acts as a switcher so that Fuchsia can use the zxtest
backend for running our tests as part of their build.

FuchsiaTest.h is for including fuchsia's zxtest library and anything
else needed to make the tests work under fuchsia (currently just
undefining the isascii macro for the test).

Downstream users, please fix your build instead of reverting.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D94625