selftests/bpf: Fix test_verifier after introducing resolve_pseudo_ldimm64

Commit 4976b718 ("bpf: Introduce pseudo_btf_id") switched
the order of check_subprogs() and resolve_pseudo_ldimm() in
the verifier. Now an empty prog expects to see the error "last
insn is not an the prog of a single invalid ldimm exit or jmp"
instead, because the check for subprogs comes first. It's now
pointless to validate that half of ldimm64 won't be the last
instruction.

Tested:
 # ./test_verifier
 Summary: 1129 PASSED, 537 SKIPPED, 0 FAILED
 and the full set of bpf selftests.

Fixes: 4976b718 ("bpf: Introduce pseudo_btf_id")
Signed-off-by: Hao Luo <haoluo@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201007022857.2791884-1-haoluo@google.com