selftests/bpf: Add tests for rbtree API interaction in sleepable progs

Confirm that the following sleepable prog states fail verification:
  * bpf_rcu_read_unlock before bpf_spin_unlock
     * RCU CS will last at least as long as spin_lock CS

Also confirm that correct usage passes verification, specifically:
  * Explicit use of bpf_rcu_read_{lock, unlock} in sleepable test prog
  * Implied RCU CS due to spin_lock CS

None of the selftest progs actually attach to bpf_testmod's
bpf_testmod_test_read.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/r/20230821193311.3290257-8-davemarchevsky@fb.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>