selftests/bpf: Check for PREEMPTION instead of PREEMPT

selftests/bpf: Check for PREEMPTION instead of PREEMPT

CONFIG_PREEMPT is a preemtion model the so called "Low-Latency Desktop".
A different preemption model is PREEMPT_RT the so called "Real-Time".
Both implement preemption in kernel and set CONFIG_PREEMPTION.
There is also the so called "LAZY PREEMPT" which the "Scheduler
controlled preemption model". Here we have also preemption in the kernel
the rules are slightly different.

Therefore the testsuite should not check for CONFIG_PREEMPT (as one
model) but for CONFIG_PREEMPTION to figure out if preemption in the
kernel is possible.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/20241119161819.qvEcs-n_@linutronix.de
Signed-off-by: Alexei Starovoitov <ast@kernel.org>