selftests/bpf: Print unexpected output on fail

This makes it easier to debug off-hand when the error message isn't
exactly as expected.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/test_verifier.c

@@ -11291,7 +11291,8 @@ static void do_test_single(struct bpf_test *test, bool unpriv,
 			goto fail_log;
 		}
 		if (!strstr(bpf_vlog, expected_err) && !reject_from_alignment) {
-			printf("FAIL\nUnexpected error message!\n");
+			printf("FAIL\nUnexpected error message!\n\tEXP: %s\n\tRES: %s\n",
+			      expected_err, bpf_vlog);
 			goto fail_log;
 		}
 	}