Shutdown executorservice when computing liveness in tests

Change-Id: I4058797c4f45d1f4ef6b36c78e4417a7df8188f8
diff --git a/src/test/java/com/android/tools/r8/TestBase.java b/src/test/java/com/android/tools/r8/TestBase.java
index eb7769e..bad14ef 100644
--- a/src/test/java/com/android/tools/r8/TestBase.java
+++ b/src/test/java/com/android/tools/r8/TestBase.java
@@ -815,6 +815,7 @@
     EnqueuerResult enqueuerResult =
         EnqueuerFactory.createForInitialTreeShaking(appView, executor, subtypingInfo)
             .traceApplication(rootSet, executor, Timing.empty());
+    executor.shutdown();
     // We do not run the tree pruner to ensure that the hierarchy is as designed and not modified
     // due to liveness.
     return appView.setAppInfo(enqueuerResult.getAppInfo());