Gnutls no longer accepts certificates whose notbefore field is a non-digits string

issues #870 (closed) describes gnutls accepting a certificate whose notbefore field is a non-digits string. According to RFC5280, gnutls should reject such certificates. the time in the certificate must be a string of numbers.

To solve the issue，add a check for the value of the notbefore or notafter field in the function _gnutls_utcTime2gtime. check whether the value of the notbefore or notafter field contains non-digits characters, and reject certificates whose notbefore or notafter include non-digits characters.

We also add file tests/cert-tests/cert-non-digits-time. This file is used to check if gnutls accepts a certificate whose time field is a non-digits string. this issue was found by our semantic differential fuzzer.

Resolves: #870 (closed)

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated / NEWS entry present (for non-trivial changes)
• CI timeout is 2h or higher (see Settings/CICD/General pipelines/Timeout)

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code