tests: fix unsigned/signed comparison in misc-free-perf.cc

thread_allocator::next_core is compared against sched::cpus.size().
GCC 4.9 doesn't like the fact that the former is signed and the latter
is unsigned.

Reviewed-by: Tomasz Grabiec <tgrabiec@gmail.com>
Signed-off-by: Paweł Dziepak <pdziepak@quarnos.org>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>