Fix race condition in releasedCallback test

WebViewFunctor::~Handle() does a post() to renderthread for
destruction. So the test needs to fence after the last sync to ensure
the posted message is processed before proceeding.

Fixes: 173156648
Test: hwuiunit --gtest_repeat=10000 --gtest_filter=RenderNode.releasedCallback --gtest_break_on_failure
Change-Id: I71d21cde43db3f689788e1d0685b30f9e6fdec15

libs/hwui/tests/unit/RenderNodeTests.cpp

@@ -264,6 +264,8 @@ TEST(RenderNode, releasedCallback) {
     TestUtils::runOnRenderThreadUnmanaged([&] (RenderThread&) {
         TestUtils::syncHierarchyPropertiesAndDisplayList(node);
     });
+    // Fence on any remaining post'd work
+    TestUtils::runOnRenderThreadUnmanaged([] (RenderThread&) {});
     EXPECT_EQ(2, counts.sync);
     EXPECT_EQ(1, counts.destroyed);
 }