selftests/bpf: Handle LOAD_REJECT test cases

In preparation to move test cases from bpf/test_sock_addr.c that expect
LOAD_REJECT, this patch adds expected_attach_type and extends load_fn to
accept an expected attach type and a flag indicating whether or not
rejection is expected.
Signed-off-by: Jordan Rife <jrife@google.com>
Link: https://lore.kernel.org/r/20240510190246.3247730-4-jrife@google.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>