[2.x] Add testForkedParallelism setting for forked test thread count (#8453)

**Problems**

When running forked tests, sbt uses `Runtime.getRuntime().availableProcessors()` to determine the thread pool size, ignoring `concurrentRestrictions`. This is inconsistent with non-forked parallel tests.

**Expectations**

Users should be able to control the number of parallel test threads in forked mode, similar to how `concurrentRestrictions` works for non-forked tests.

**Notes**

Added a new setting `testForkedParallelism` that allows explicit control:

```scala
testForkedParallelism := Some(2)  // Use 2 threads
testForkedParallelism := None     // Use availableProcessors() (default)
```

worker/src/main/java/sbt/internal/worker1/ForkTestMain.java

@@ -311,12 +311,13 @@ public class ForkTestMain {
       this.originalOut.flush();
     }
 
-    private ExecutorService executorService(final boolean parallel) {
+    private ExecutorService executorService(final boolean parallel, final Integer parallelism) {
       if (parallel) {
-        final int nbThreads = Runtime.getRuntime().availableProcessors();
+        final int nbThreads =
+            (parallelism != null && parallelism > 0)
+                ? parallelism
+                : Runtime.getRuntime().availableProcessors();
         logDebug("Create a test executor with a thread pool of " + nbThreads + " threads.");
-        // more options later...
-        // TODO we might want to configure the blocking queue with size #proc
         return Executors.newFixedThreadPool(nbThreads);
       } else {
         logDebug("Create a single-thread test executor");
@@ -326,7 +327,7 @@ public class ForkTestMain {
 
     private void runTests(TestInfo info, ClassLoader classLoader) throws Exception {
       Thread.currentThread().setContextClassLoader(classLoader);
-      final ExecutorService executor = executorService(info.parallel);
+      final ExecutorService executor = executorService(info.parallel, info.parallelism);
       final TaskDef[] tests = info.taskDefs.toArray(new TaskDef[] {});
       final int nFrameworks = info.testRunners.size();
       final Logger[] loggers = {remoteLogger(info.ansiCodesSupported)};

worker/src/main/java/sbt/internal/worker1/TestInfo.java

@@ -33,6 +33,7 @@ public class TestInfo implements Serializable {
   public final RunInfo.NativeRunInfo nativeRunInfo;
   public final boolean ansiCodesSupported;
   public final boolean parallel;
+  public final Integer parallelism;
   public final ArrayList<TaskDef> taskDefs;
   public final ArrayList<TestRunner> testRunners;
 
@@ -42,6 +43,7 @@ public class TestInfo implements Serializable {
       RunInfo.NativeRunInfo nativeRunInfo,
       boolean ansiCodesSupported,
       boolean parallel,
+      Integer parallelism,
       ArrayList<TaskDef> taskDefs,
       ArrayList<TestRunner> testRunners) {
     this.jvm = jvm;
@@ -49,6 +51,7 @@ public class TestInfo implements Serializable {
     this.nativeRunInfo = nativeRunInfo;
     this.ansiCodesSupported = ansiCodesSupported;
     this.parallel = parallel;
+    this.parallelism = parallelism;
     this.taskDefs = taskDefs;
     this.testRunners = testRunners;
   }