selftests/bpf: __not_msg() tag for test_loader framework

selftests/bpf: __not_msg() tag for test_loader framework

This patch adds tags __not_msg(<msg>) and __not_msg_unpriv(<msg>).
Test fails if <msg> is found in verifier log.

If __msg_not() is situated between __msg() tags framework matches
__msg() tags first, and then checks that <msg> is not present in a
portion of a log between bracketing __msg() tags.
__msg_not() tags bracketed by a same __msg() group are effectively
unordered.

The idea is borrowed from LLVM's CheckFile with its CHECK-NOT syntax.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250918-callchain-sensitive-liveness-v3-11-c3cd27bacc60@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>