Wait for callbacks before exiting test

Bug: 152014889
Test: build, boot, libgui_test
Change-Id: I9673933f81b903081d265ed22d0afb71008a6541
diff --git a/libs/gui/tests/BLASTBufferQueue_test.cpp b/libs/gui/tests/BLASTBufferQueue_test.cpp
index a87ccd6..d929a59 100644
--- a/libs/gui/tests/BLASTBufferQueue_test.cpp
+++ b/libs/gui/tests/BLASTBufferQueue_test.cpp
@@ -722,5 +722,8 @@
     ASSERT_EQ(2, events->frameNumber);
     ASSERT_EQ(requestedPresentTimeB, events->requestedPresentTime);
     ASSERT_GE(events->postedTime, postedTimeB);
+
+    // wait for any callbacks that have not been received
+    adapter.waitForCallbacks();
 }
 } // namespace android