Combine push and PR actions

This reduces duplication, with the tradeoff of having some steps in the
linter job that never get executed in one or the other situation.

.github/workflows/checks-pr.yml → .github/workflows/checks.yml

@@ -1,10 +1,14 @@
-name: Required PR checks
+name: Checks
 
-on: pull_request
+on:
+  - push:
+      branches:
+        main
+  - pull_request
 
 jobs:
   lint:
-    name: Lint
+    name: Lint tests
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
@@ -21,14 +25,21 @@ jobs:
           python -m pip install --upgrade pip
           pip install -r tools/lint/requirements.txt
 
+      - name: Lint all tests
+        if: github.event_name == 'push'
+        run: ./tools/lint/lint.py test/
+
       - name: Identify new or changed tests
         id: changed_tests
+        if: github.event_name == 'pull_request'
         uses: tj-actions/changed-files@v45
         with:
           files: test/
 
       - name: Lint new or changed tests
-        if: steps.changed_tests.outputs.any_changed == 'true'
+        if: |
+          github.event_name == 'pull_request' &&
+            steps.changed_tests.outputs.any_changed == 'true'
         env:
           CHANGED: ${{ steps.changed_tests.outputs.all_changed_files }}
         run: |
@@ -39,7 +50,7 @@ jobs:
           ./tools/lint/lint.py $CHANGED 2> >(tee errors.out >&2)
 
       - name: Create annotations in the PR
-        if: failure()
+        if: failure() && github.event_name == 'pull_request'
         run: |
           awk -F': ' <errors.out \
             "{ sub(\"$(pwd)/\", \"\"); printf \"::error file=%s,line=1::%s\n\", \$1, \$2 }"