build: replace IN_WESTON with UNIT_TEST

Remove IN_WESTON in favour of the already defined UNIT_TEST which is
used to modify a compilation unit to expose more functions for unit
tests to prod at.

Originally IN_WESTON meant that compilation unit was being compiled for
use in the Weston compositor, but it probably never really did anything
more than change what WL_EXPORT means in matrix.c.

This patch not only simplifies the logic, but it fixes the Meson build
of test-matrix: IN_WESTON was defined there even when matrix.c was being
built outside of Weston, which caused it to depend on libwayland
headers, which were not included in the Meson build of test-matrix.
Test-matrix has no reason to depend in libwayland in any way, so this
patch fixes that.

Reported-by: Greg V <greg@unrelenting.technology>
Signed-off-by: Pekka Paalanen <pq@iki.fi>