Fix ReviewAppChildPipeline to properly detect variable

What does this MR do and why?

Variables are passed as [{ 'key' => 'actual_key', 'variable' => 'actual_value' }] in the event payload. Detection should take that inaccount.

I also added a default

    it 'returns true' do
      expect(described_class.match?(event)).to be_truthy
    end

test case for each config classes as the default/positive case wasn't actually tested!

Expected impact & dry-runs

We should start to see notifications coming in the #review-apps-broken channel.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (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