Instead of relying on Meson setting up environment so that Weston and tests
find all their files, build those values into the tests. This way one can
execute a test program successfully wihtout Meson, simply by running it.

The old environment variables are still honoured if set. This might change in
the future.

Baking the source or build directory paths into the tests should not regress
reproducible builds, because the binaries where test-config.h values are used
will not be installed.

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