bpf: selftest: Add verifier tests for <8-byte scalar spill and refill

This patch adds a few verifier tests for <8-byte spill and refill.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210922004953.627183-1-kafai@fb.com