libtest: Install the library, headers and pkg-config file

This opens up the possibility of using the testing framework in external
modules, too.

Requested-by: Viktor Tusa <tusavik@gmail.com>
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge pull request #133 from algernon/f/libtest-install

libtest: Install the library, headers and pkg-config file