selftests/bpf: Support checks against a regular expression

selftests/bpf: Support checks against a regular expression

Add support for __regex and __regex_unpriv macros to check the test
execution output against a regular expression. This is similar to __msg
and __msg_unpriv, however those expect do substring matching.

Signed-off-by: Cupertino Miranda <cupertino.miranda@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20240617141458.471620-2-cupertino.miranda@oracle.com