tests: allow running make check without make install

tests: allow running make check without make install

desktop shell and weston keyboard both refer to themselves prefixed by
LIBEXECDIR, however this is only valid once installed.  make check will
currently either fail or run pre-existing versions.

This patch adds a way to override that location by setting the env var
WESTON_BUILD_DIR - which is then set by the test env script so make check
will test the versions in the build directory regardless of whether they're
installed or not.

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>