tests: destroy client in keyboard-test

Fixes all the leaks reported by ASan in this test.

The manual keyboard release in
keyboard_timestamps_stop_after_client_releases_wl_keyboard is slightly
awkward as we need to open-code a part of input_destroy() to avoid
double-freeing keyboard->wl_keyboard.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>