Add unit test for onBufferFreedListener

As a follow up to cover new functionality introduced in ag/1998938

Bug: 35114769
Test: run libgui_test with --gtest_filter='BufferItemConsumerTest.*'

Change-Id: I461dc88eac84e165f80a1838b63bfd6bb22a56d7
diff --git a/libs/gui/tests/Android.bp b/libs/gui/tests/Android.bp
index 4492a08..3a99147 100644
--- a/libs/gui/tests/Android.bp
+++ b/libs/gui/tests/Android.bp
@@ -8,6 +8,7 @@
     clang: true,
 
     srcs: [
+        "BufferItemConsumer_test.cpp",
         "BufferQueue_test.cpp",
         "CpuConsumer_test.cpp",
         "FillBuffer.cpp",