selftests/bpf: Some more atomic tests

Some new verifier tests that hit some important gaps in the parameter
space for atomic ops.

There are already exhaustive tests for the JIT part in
lib/test_bpf.c, but these exercise the verifier too.

Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211015093318.1273686-1-jackmanb@google.com