A convenience wrapper name `add_libc_test` is also added which adds both
a unit test and a hermetic test. The ctype tests have been switched over
to use add_libc_test.
Reviewed By: jhuber6
Differential Revision: https://reviews.llvm.org/D148756
This part of the effort to make all test related pieces into the `test`
directory. This helps is excluding test related pieces in a straight
forward manner if LLVM_INCLUDE_TESTS is OFF. Future patches will also move
the MPFR wrapper and testutils into the 'test' directory.