test/rgw: init CephContext in unittest_rgw_cors

unittest_rgw_cors used GMock::Main without global_init, so dout() in
RGWCORSRule::matches() dereferenced a null g_ceph_context and segfaulted
on the first test. Link unit-main like unittest_rgw_compression.

Assisted-by: Cursor:composer-2.5-fast
Signed-off-by: ramin.najarbashi <ramin.najarbashi@gmail.com>
Co-authored-by: Cursor <cursoragent@cursor.com>