[libc] Remove *TestMain libraries and combine them with the main test libraries.

There are not tests currently which use the main test framework but not
the `main` function from LibcTestMain.cpp. So, this change essentially
simplifies by merging the *TestMain libraries with the main test
libraries.

Reviewed By: michaelrj, jhuber6

Differential Revision: https://reviews.llvm.org/D150698