selftests/bpf: Update spinlock selftest

Make updates in preparation for adding more test cases to this selftest:
- Convert from CHECK_ to ASSERT macros.
- Use BPF skeleton
- Fix typo sping -> spin
- Rename spinlock.c -> spin_lock.c

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20221118015614.2013203-21-memxor@gmail.com


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