test: don't leave open read views after vinyl/iterator

An open iterator may disrupt the following test run, because it may
prevent dump/compaction from purging stale rows. In particular,
iterators left by vinyl/iterator result in the following test failure:

 | --- vinyl/deferred_delete.result        Mon Feb 11 19:14:01 2019
 | +++ vinyl/deferred_delete.reject        Fri Mar 15 16:21:11 2019
 | @@ -155,7 +155,7 @@
 |  ...
 |  pk:stat().rows -- 5 new REPLACEs
 |  ---
 | -- 5
 | +- 10
 |  ...
 |  i1:stat().rows -- 10 old REPLACE + 5 new REPLACEs + 10 deferred DELETEs
 |  ---

Fix this by calling the Lua garbage collector to delete all dangling
iterators in the end of vinyl/iterator test.

Closes #3862