E2E test for Push Rule: Reject unsigned commits

Docs: https://docs.gitlab.com/ee/push_rules/push_rules.html

Note: There are several push rules, so it might be best to work on their tests together:

• #89 (closed)
• #90 (closed)
• #91 (closed)
• #92 (closed)
• #93 (closed)
• #94 (closed)
• #95 (closed)
• #96 (closed)
• #97 (closed)
• #98 (closed)

Related epic: gitlab-org&1669