selftests/bpf: Expand coverage of preempt tests to sleepable kfunc

selftests/bpf: Expand coverage of preempt tests to sleepable kfunc

For preemption-related kfuncs, we don't test their interaction with
sleepable kfuncs (we do test helpers) even though the verifier has
code to protect against such a pattern. Expand coverage of the selftest
to include this case.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20241204030400.208005-7-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>