Introduce Checkstyle integration test

The new `integration-tests/checkstyle-10.12.4.sh` script can be executed
locally or by adding a comment containing the string `/integration-test`
to a pull request.

The integration test comprises a shell script that:
1. Checks out a Checkstyle release tag.
2. Applies a small patch to enable Error Prone Support.
3. Runs the build in Error Prone patch mode, until no further changes
   are applied.
4. Validates that:
   - The build (including tests) still passes.
   - The set of changes matches a pre-recorded diff.
   - The set of Error Prone Support-emitted warnings matches a
     pre-recorded list.

The script also has a `--sync` mode using which the expected diff and
warnings can be updated. Combined, the script makes it easy to assess
and track the impact of changes to Error Prone Support in relation to
the Checkstyle code base.

Over time this setup will be generalized to include integration tests
against other code bases.

.github/workflows/run-integration-tests.yml

@@ -0,0 +1,32 @@
+# If requested by means of a pull request comment, runs integration tests
+# against the project, using the code found on the pull request branch.
+# XXX: Generalize this to a matrix build of multiple integration tests,
+# possibly using multiple JDK or OS versions.
+name: "Integration tests"
+on:
+  issue_comment:
+    types: [ created ]
+permissions:
+  contents: read
+jobs:
+  run-integration-tests:
+    name: On-demand integration test
+    if: |
+      github.event.issue.pull_request && contains(github.event.comment.body, '/integration-test')
+    runs-on: ubuntu-22.04
+    steps:
+      - name: Check out code
+        uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
+        with:
+          persist-credentials: false
+          ref: refs/pull/${{ github.event.issue.number }}/head
+      - name: Set up JDK
+        uses: actions/setup-java@0ab4596768b603586c0de567f2430c30f5b0d2b0 # v3.13.0
+        with:
+          java-version: 17.0.8
+          distribution: temurin
+          cache: maven
+      - name: Install project to local Maven repository
+        run: mvn -T1C install -DskipTests -Dverification.skip
+      - name: Run integration test
+        run: xvfb-run ./integration-tests/checkstyle-10.12.4.sh

.gitignore

@@ -9,3 +9,6 @@
 target
 *.iml
 *.swp
+
+# The Git repositories checked out by the integration test framework.
+integration-tests/.repos