Move defragmenter benchmarks to benchmarks

Move defragmenter benchmarks to benchmarks

This moves the defragmenter benchmarks to ep_engine_benchmarks because
they can take ~18 seconds under ASAN. It also makes sense to group all
the benchmarks together.

To enable this move, this commit also puts the defragmenter test in to a
header file so that it can continued to be used by the defragmenter
tests which are not benchmarks.

Change-Id: I69f8da02c07e6469b9c80fbe06507c80d866ac7f
Reviewed-on: http://review.couchbase.org/76476
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>