[libc] Make hermetic test depend on the unit test if it exists.

We want to do this so that build system like ninja don't end up running
the hermetic and unit tests in parallel. Running in parallel can cause
problems for tests which read/write disk files as the hermetic and unit
tests can end up stepping on each other.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D151291