Revert "Revert "ART: Add ThreadGroup API support""

This reverts commit 87071bfb6c1b708bdfa2a5f91d4744667b3a0443.

Add an ObjectLock, which corresponds to the synchronized(this)
implementation on the Java side. Wait for the expected five
child threads in the root group before running the actual
child test.

Bug: 31455788
Change-Id: Ib7a065d6a11f06f0325e3a8db040629f3ca69407
Test: m test-art-host-run-test-925-threadgroups
diff --git a/test/925-threadgroups/run b/test/925-threadgroups/run
new file mode 100755
index 0000000..4379349
--- /dev/null
+++ b/test/925-threadgroups/run
@@ -0,0 +1,19 @@
+#!/bin/bash
+#
+# Copyright 2016 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+./default-run "$@" --experimental agents \
+                   --experimental runtime-plugins \
+                   --jvmti