selftests: bpf: Don't run sk_lookup in verifier tests

sk_lookup doesn't allow setting data_in for bpf_prog_run. This doesn't
play well with the verifier tests, since they always set a 64 byte
input buffer. Allow not running verifier tests by setting
bpf_test.runs to a negative value and don't run the ctx access case
for sk_lookup. We have dedicated ctx access tests so skipping here
doesn't reduce coverage.

Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210303101816.36774-6-lmb@cloudflare.com

tools/testing/selftests/bpf/verifier/ctx_sk_lookup.c

@@ -239,6 +239,7 @@
 	.result = ACCEPT,
 	.prog_type = BPF_PROG_TYPE_SK_LOOKUP,
 	.expected_attach_type = BPF_SK_LOOKUP,
+	.runs = -1,
 },
 /* invalid 8-byte reads from a 4-byte fields in bpf_sk_lookup */
 {