ci: Run 3 concurrent test processes across 3 stress-test shards

Increase resource pressure to reproduce intermittent electrs failures.
Each of the 3 shards runs 5 iterations with 3 concurrent cargo test
processes, for 45 total test runs with up to 9 simultaneous processes.

AI tools were used in preparing this commit.

Author: joostjager

.github/workflows/rust.yml

@@ -87,6 +87,10 @@ jobs:
           RUSTFLAGS="--cfg no_download --cfg cycle_tests" cargo test --features uniffi
 
   stress-test:
+    strategy:
+      fail-fast: false
+      matrix:
+        shard: [1, 2, 3]
     runs-on: ubuntu-latest
     steps:
       - name: Checkout source code
@@ -119,11 +123,21 @@ jobs:
           echo "ELECTRS_EXE=$( pwd )/bin/electrs-${{ runner.os }}-${{ runner.arch }}" >> "$GITHUB_ENV"
       - name: Build integration tests
         run: RUSTFLAGS="--cfg no_download --cfg cycle_tests" cargo test --test integration_tests_rust --no-run
-      - name: Stress-test integration tests (10 iterations)
+      - name: Stress-test integration tests (5 iterations, 3 concurrent processes)
         run: |
-          for i in $(seq 1 10); do
-            echo "=== Iteration $i ==="
-            RUSTFLAGS="--cfg no_download --cfg cycle_tests" cargo test --test integration_tests_rust -- --nocapture 2>&1 || { echo "FAILED on iteration $i"; exit 1; }
+          for i in $(seq 1 5); do
+            echo "=== Iteration $i (shard ${{ matrix.shard }}) ==="
+            for j in 1 2 3; do
+              RUSTFLAGS="--cfg no_download --cfg cycle_tests" cargo test --test integration_tests_rust -- --nocapture > /tmp/stress-${j}.log 2>&1 &
+              pids[${j}]=$!
+            done
+            for j in 1 2 3; do
+              if ! wait ${pids[${j}]}; then
+                echo "FAILED: process $j on iteration $i (shard ${{ matrix.shard }})"
+                cat /tmp/stress-${j}.log
+                exit 1
+              fi
+            done
           done
 
   doc: