Add temporary cluster stress job

Six parallel cluster-suite runs per push, to measure the flake rate of
TestFallbackToSlave* before and after a fix. Removed before merge.

Author: g7r

.github/workflows/ci.yml

@@ -59,3 +59,45 @@ jobs:
 
     - name: Build
       run: make ${{ matrix.stage }}
+
+  # TEMPORARY: flake-rate measurement, remove before merge.
+  cluster-stress:
+
+    runs-on: ubuntu-latest
+    timeout-minutes: 25
+
+    strategy:
+      fail-fast: false
+      matrix:
+        run: [1, 2, 3, 4, 5, 6]
+
+    steps:
+
+    - name: Checkout
+      uses: actions/checkout@v4
+
+    - name: Set up Go
+      uses: actions/setup-go@v5
+      with:
+        go-version: "1.24"
+
+    - name: Cache go modules
+      uses: actions/cache@v4
+      with:
+        path: ~/go/pkg/mod
+        key: ${{ runner.os }}-go-${{ hashFiles('**/go.sum') }}
+        restore-keys: |
+          ${{ runner.os }}-go-
+
+    - name: Cache redis build
+      uses: actions/cache@v4
+      with:
+        path: |
+          /tmp/redis-server
+        key: ${{ runner.os }}-redis-${{ hashFiles('Makefile') }}
+
+    - name: Install redis
+      run: make /tmp/redis-server/redis-server
+
+    - name: Build
+      run: make testcluster