Make runForTesting use InternalOptions.numberOfThreads again.

Introduced accidentally in 9aab4f8.

Bug:
Change-Id: I9feff3371e62d2a39654f3c1b79a9176b18db158
diff --git a/src/main/java/com/android/tools/r8/D8.java b/src/main/java/com/android/tools/r8/D8.java
index 40dea06..4a26889 100644
--- a/src/main/java/com/android/tools/r8/D8.java
+++ b/src/main/java/com/android/tools/r8/D8.java
@@ -122,7 +122,7 @@
 
   static void runForTesting(AndroidApp inputApp, InternalOptions options)
       throws IOException, CompilationException {
-    ExecutorService executor = ThreadUtils.getExecutorService(ThreadUtils.NOT_SPECIFIED);
+    ExecutorService executor = ThreadUtils.getExecutorService(options);
     try {
       run(inputApp, options, executor);
     } finally {