tests: vhost-user-bridge: Avoid using hardcoded /tmp

This case was written to use hardcoded /tmp directory for temporary
files. Update to use g_file_open_tmp() for a portable implementation.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20220925113032.1949844-25-bmeng.cn@gmail.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>