meson: set WESTON_DATA_DIR for tests

It seems WESTON_DATA_DIR was missed. If you have already installed Weston, then
the files will be found in the install location, but if not, they were not
found at all.

This caused the xwayland test to SEGV the compositor in
weston_wm_window_create_frame() when frame_crate() returned NULL.

This patch fixes the test suite only.

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