fix: Don’t test draft PRs. Don’t do multiple “Test” runs at once. (#108)

Part of
github/continuous-ai-for-accessibility#31
(Hubber access only).

This PR prevents a ‘thundering herd’-like problem: A PR would trigger
the “Test” workflow which, in turn, opens 6 more PRs which, in turn,
each trigger “Test” and open more PRs, etc. It does this by
early-exiting for draft PRs (like the 6). (Additionally, these cascading
“Test” runs would always fail: The PRs fix issues, so assertions re:
number of known/expected findings in “site-with-issues” fail.)

This PR also modifies workflow triggers to reduce instances where the
same code is tested twice. For example, since this repo enforces PRs,
`on.push.main` was redundant. This conserves our request quota.

This PR also prevents multiple “Test” workflows from running at once
(which would cause cache collisions). It continues to allow multiple
sites to be scanned concurrently _within_ a workflow run (which does not
cause cache collisions).

Author: smockle

.github/workflows/test.yml

@@ -2,21 +2,27 @@ name: Test
 on:
   pull_request:
     types:
+      - opened
+      - reopened
       - synchronize
-  push:
-    branches:
-      - main
+      - ready_for_review
   workflow_dispatch:
 
 permissions:
   contents: write
   issues: write
   pull-requests: write
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: false # Allow previous workflows’ clean-up steps to complete
+
 jobs:
   test:
     name: Test
     runs-on: ubuntu-latest
+    # Run if triggered manually, or for a non-draft PR
+    if: ${{ github.event_name != 'pull_request' || github.event.pull_request.draft == false }}
     strategy:
       matrix:
         site: [ "sites/site-with-errors" ]