tests/kernel/smp: Fix cases for !SCHED_IPI_SUPPORTED

Obviously the test of the feature won't work if we don't have an IPI.
And there were two threads that spawned threads that enter busy loops,
expecting to be able to abort them from another CPU.  That doesn't
work[1] without an IPI.  Just skip these cases rather than trying to
kludge up some kind of abort signal.

[1] Rather, it does work, it just takes infinite time for these
    particular test cases.  Eventually the CPU would be expected to
    receive some other interrupt like a timeout, which would work to
    abort the running thread.  But no such timer was registered.

Signed-off-by: Andy Ross <andyross@google.com>