[tests] Add test_egl_configs to list available EGL configurations

Allows easier debugging of which EGL configurations are available for
any given hardware / driver combination. Tries to print as much in
symbolic EGL_* constants as possible where it makes sense.

Signed-off-by: Thomas Perl <m@thp.io>
Merge pull request #162 from thp/egl-configs-test

[tests] Add test_egl_configs to list available EGL configurations