Limit E2E test parallelism

The current Github runner is low on resources, so running many E2E tests
can make tests more flaky then necessary. For now, it seems, we need to
leave with this.

.github/workflows/e2e.yml

@@ -60,26 +60,57 @@ jobs:
             export K6_BROWSER_EXECUTABLE_PATH=/usr/bin/google-chrome
           fi
           export K6_BROWSER_HEADLESS=true
+
+          # run only two E2E tests in parallel
+          # the current runner is low on resources
+          max_concurrent_jobs=2
+          current_jobs=0
+
           declare -a pids
           declare -a output_files
 
+          launch_job() {
+            local f="$1"
+
+            local output_file="output_$(basename "$f" .js).txt"
+            output_files+=("$output_file")
+
+            # Run the test in the background
+            ./k6extension run "$f" > "$output_file" 2>&1 &
+            local pid=$!
+            
+            pids+=($pid)
+            current_jobs=$((current_jobs + 1))
+          }
+
           for f in examples/*.js; do
             if [ "$f" == "examples/hosts.js" ] && [ "$RUNNER_OS" == "Windows" ]; then
               echo "skipping $f on Windows"
               continue
             fi
-            output_file="output_$(basename "$f" .js).txt"
-            output_files+=("$output_file")
-            ./k6extension run -q "$f" > "$output_file" 2>&1 &
-            pids+=($!)
+
+            launch_job "$f"
+
+            # If we've reached the maximum number of concurrent jobs, wait for one to finish
+            if [ "$current_jobs" -ge "$max_concurrent_jobs" ]; then
+              wait "${pids[0]}"
+              unset "pids[0]"
+              pids=("${pids[@]}")
+              current_jobs=$((current_jobs - 1))
+            fi
           done
 
-          # Wait for each background job to finish and display its output
-          for i in "${!pids[@]}"; do
-            wait "${pids[$i]}"
-            echo "Output for job ${pids[$i]} (from file ${output_files[$i]}):"
-            cat "${output_files[$i]}"
+          # Wait for all remaining jobs to finish
+          for pid in "${pids[@]}"; do
+            wait "$pid"
           done
+
+          # Output the results
+          for output_file in "${output_files[@]}"; do
+            echo "Output from file $output_file:"
+            cat "$output_file"
+          done
+
       - name: Check screenshot
         if: matrix.go != 'tip' || matrix.platform != 'windows-latest'
         # TODO: Do something more sophisticated?