added tests: kv_copy_var op

+  kv_copy_var_test
 * verify single entry from source bnode can be put into destination

+ kv_copy_var_nentry_test
 * verify n entries from source bnode can be copied into dest at various offsets
 *   -1, copy n/2-entries into empty destination
 *   -2, append n-entries into occupied destination
 *   -3, prepend n-entries into occupied destination

Change-Id: Iee6b79a6cae6a78b85848e2c16e0dea5a366ea3e
Merge "added tests: kv_copy_var op"