Add two new CMakeLists, /cmake/target_api/{gl,gles2}/CMakeLists.txt, each
of which builds Piglit with its respective API. (Of course,
gles2/CMakeLists.txt adds no targets because Piglit does not yet support
building with GLES2.)

The toplevel CMakeLists no longer adds subdirectory /tests; that is now
the responsiblity of the API-specific CMakeLists.

Note: This belongs to a series that adds GLES2 support to Piglit.
Signed-off-by: Chad Versace <chad.versace@intel.com>