[libc] Add 'UNIT_TEST_ONLY' and 'HERMETIC_TEST_ONLY' to 'add_libc_test'

The `add_libc_test` option allows us to enable both kinds of tests in a
single option. However, some tests cannot be made hermetic with the
current approach. Such as tests that rely on system utilities or
libraries. This patch adds two options `UNIT_TEST_ONLY` and
`HERMETIC_TEST_ONLY` to offer more fine-grained control over which
version gets built. This makes it explicit which version a test supports
and why.

Depends on D149662

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D149691