Refactor x509 signature tests

What does this MR do and why?

Problem:

Tests from files:

• spec/lib/gitlab/x509/signature_sigstore_spec.rb
• spec/lib/gitlab/x509/signature_spec.rb

have a lot of duplication and are very difficult to modify.

Solution:

• Remove duplication
• Extract common parts into shared_examples

MR acceptance checklist

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Related to #553481