[libc++] Remove unused functions and minor features of the test suite

This commit removes minor features of the test suite that I've never
seen used and that are basically just a maintenance burden:

- color_diagnostics: Diagnostics are colored by default when running
  from a terminal, and not colored otherwise. This is the right behavior.
  Being able to tweak this has minor value, and could be achieved by
  modifying the %{compile_flags} instead if absolutely needed.

- ccache: This can be achieved by using a wrapper for the %{cxx}
  substitution.

- _dump_macros_verbose is just a dead function now.