tests: fix memory leak in proxy-test

When running tests with ASan, proxy-test fails at the proxy_tag test:

    ==27843==ERROR: LeakSanitizer: detected memory leaks

    Direct leak of 32 byte(s) in 1 object(s) allocated from:
        #0 0x7f65a732dada in __interceptor_malloc /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cc:144
        #1 0x7f65a71cb3ea in wl_display_add_protocol_logger src/wayland-server.c:1813
        #2 0x557c640c0980 in proxy_tag tests/proxy-test.c:104
        #3 0x557c640c1159 in run_test tests/test-runner.c:153
        #4 0x557c640c1e2e in main tests/test-runner.c:337
        #5 0x7f65a6ea0ee2 in __libc_start_main (/usr/lib/libc.so.6+0x26ee2)

    SUMMARY: AddressSanitizer: 32 byte(s) leaked in 1 allocation(s).

Destroying the logger fixes the leak.

Signed-off-by: Simon Ser <contact@emersion.fr>
Fixes: 493ab79b ("proxy: Add API to tag proxy objects")