Add test that checks duplicate meta_seq entries

Add a quick test of the index items to make sure that rapid inode
updates don't create duplicate meta_seq items.

Signed-off-by: Zach Brown <zab@versity.com>

tests/golden/simple-inode-index

@@ -7,3 +7,4 @@ found second
 == changing metadata must increase meta seq
 == changing contents must increase data seq
 == make sure dirtying doesn't livelock walk
+== concurrent update attempts maintain single entries