X509: add tests for purpose code signing in verify application

X509: add tests for purpose code signing in verify application

Correct configuration according to CA Browser forum:
  KU: critical,digitalSignature
  XKU: codeSiging

Note: I did not find any other document formally defining the requirements
for code signing certificates.

Some combinations are explicitly forbidden, some flags can be ignored

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18567)