test-runner: move test runner into subdirectory

Use directories, it's cool :) Now we can store in test/ only tests and
into Makefile.am we can insert only sources of the tests, because test
runner is linked to it as .la library. Moreover, it's nicer now.
client: add listener for wl_seat

Create pointer, touch and keyboard proxies in wit_client struct
tests: test if proxies for pointer, keyboard and touch are created

in wit_client_populate()
tests: add proxy test

Test creating proxy, requesting and catching events via proxy.