tests: destroy subcompositor in alpha-blending

Fixed a leak found by ASan:

Direct leak of 160 byte(s) in 2 object(s) allocated from:
    #0 0x7f511fe11518 in calloc (/lib/x86_64-linux-gnu/libasan.so.5+0xe9518)
    #1 0x7f511fc76373 in zalloc ../../git/wayland/src/wayland-private.h:232
    #2 0x7f511fc76373 in proxy_create ../../git/wayland/src/wayland-client.c:422
    #3 0x7f511fc79dcc in create_outgoing_proxy ../../git/wayland/src/wayland-client.c:651
    #4 0x7f511fc79dcc in wl_proxy_marshal_array_constructor_versioned ../../git/wayland/src/wayland-client.c:736
    #5 0x7f511fc7b17b in wl_proxy_marshal_constructor_versioned ../../git/wayland/src/wayland-client.c:873
    #6 0x5583e5348f43 in wl_registry_bind /home/pq/local/include/wayland-client-protocol.h:1165
    #7 0x5583e535140b in bind_to_singleton_global ../../git/weston/tests/weston-test-client-helper.c:1863
    #8 0x5583e5348752 in alpha_blend_monotonic ../../git/weston/tests/alpha-blending-test.c:219
    #9 0x5583e5348571 in wrapalpha_blend_monotonic ../../git/weston/tests/alpha-blending-test.c:200
    #10 0x5583e53554cc in run_test ../../git/weston/tests/weston-test-runner.c:162
    #11 0x5583e5355b6d in run_case ../../git/weston/tests/weston-test-runner.c:277
    #12 0x5583e5355913 in for_each_test_case ../../git/weston/tests/weston-test-runner.c:235
    #13 0x5583e5355df5 in testsuite_run ../../git/weston/tests/weston-test-runner.c:311
    #14 0x7f511aaaf752 in client_thread_routine ../../git/weston/tests/weston-test.c:404
    #15 0x7f511f88cfa2 in start_thread /build/glibc-vjB4T1/glibc-2.28/nptl/pthread_create.c:486

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

tests/alpha-blending-test.c

@@ -256,5 +256,6 @@ TEST(alpha_blend_monotonic)
 	wl_subsurface_destroy(sub);
 	wl_surface_destroy(surf);
 	buffer_destroy(buf);
+	wl_subcompositor_destroy(subco);
 	client_destroy(client);
 }