NCBC-751: GetSetPerformanceTests.Test_Timed_Execution is failing

Motivation
----------
Fixes test that was failing due to refactoring.

Modifications
-------------
Moved Set/Get instantiations within loop scope so that they would be
recreated on every loop.

Result
------
The test passes.

Change-Id: Ic6732e08bec43abf2677a7f97ce386307073ede5
Reviewed-on: http://review.couchbase.org/44446
Reviewed-by: Simon Baslé <simon@couchbase.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>