Add context selector to exclude tests from jobs

What does this MR do?

Adds an exclude option that allows us to specify the context in which to skip a job. It's complementary to the :only tag.

For example, this would skip the test in the review-qa-all job:

it 'is a test', exclude: { job: 'review-qa-all' }

• Extracts shared RSpec helpers (which allow tests within tests)
• Excludes restrict_by_ip_address_spec on Review Apps
• Excludes only for jobs for now (not pipelines or environments)
• Moves code for only tag to ContextSelector

Closes https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/821

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team