test plugin: run tests asyncronously

There was stupid bug that caused hanging the mutter when running actual
wayland client. Instead of running the tests in initial phase, load all
information you need, run them asynchronously when mutter becomes
idle and wait for the childs to exit.
test plugin: add wl_test implementation
tests: add plugin-test

Tests for mutter-test-plugin for mutter

XXX: backup commit, reset it later!