lockdep/selftests: Skip the softirq related tests on PREEMPT_RT

The softirq context on PREEMPT_RT is different compared to !PREEMPT_RT.
As such lockdep_softirq_enter() is a nop and the all the "softirq safe"
tests fail on PREEMPT_RT because there is no difference.

Skip the softirq context tests on PREEMPT_RT.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>