test-utils: Fix compiler warning
This fixes the following compiler warning:
In file included from /usr/include/glib-2.0/glib.h:114, from ../src/tests/test-utils.h:23, from ../src/tests/test-utils.c:22: ../src/tests/test-utils.c: In function ‘test_init’: /usr/include/glib-2.0/glib/glib-autocleanups.h:28:3: warning: ‘basename’ may be used uninitialized in this function [-Wmaybe-uninitialized]