Call JNI_OnUnload when class loaders get collected

Added test case to 141-class-unload.

Bug: 22720414
Change-Id: I0575fae72521520a17587e8b0088bf8112705ad8
diff --git a/test/141-class-unload/expected.txt b/test/141-class-unload/expected.txt
index be2671e..124398f 100644
--- a/test/141-class-unload/expected.txt
+++ b/test/141-class-unload/expected.txt
@@ -7,3 +7,6 @@
 null
 loader null false
 loader null false
+JNI_OnLoad called
+JNI_OnUnload called
+null