Remove no_xdg_runtime_dir_test

[6/6] Make autotests create fake input devices

This test was the only one where input() could return a nullptr. With
this test removed, autotests can now expect input() to always return a
sane valid value and are therefor simpler to write.

That test belongs in kwayland-server anyway and kwayland-server's test
suite already tests that starting without XDG_RUNTIME_DIR is a no-no
thing