test-char not concurrent with unix socket

'make check-unit' might fail when running multiple tests in parallel.

Apparently occurred on OSX CI:
https://travis-ci.org/github/philmd/qemu/jobs/662357430

Guess is same unix path used:

static SocketAddress unixaddr = {
    .type = SOCKET_ADDRESS_TYPE_UNIX,
    .u.q_unix.path = (char *)"test-char.sock",
};

Note, other tests in this file use g_dir_make_tmp().