Add a positive spec for "applicable?" in leading organization processor

What does this MR do and why?

Follow-up of postmortem: https://gitlab.com/gitlab-org/quality/contributor-success/team-task/-/issues/129#note_1217230849

Adds an event is applicable test for the Triage::LabelLeadingOrganization processor.

Why not mock the CSV in this test?

The CSV mocking is already done in https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/d08585f8f474807f59c1fb3af1a98363009173aa/spec/triage/leading_organization_detector_spec.rb#L10-15.

Triage::LeadingOrganizationDetector is the class responsible for parsing the CSV data, and finding out whether the user is part of a Leading Organization. We are mocking the call in the label_leading_organization_spec specs at https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/d08585f8f474807f59c1fb3af1a98363009173aa/spec/processor/community/label_leading_organization_spec.rb#L20-23.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review