[TEST] Reduce number of documents/terms in aggregations' RandomTests.

This test initially had three purposes:
 - duels between equivalent aggregations on significant amounts of data,
 - make sure that array growth works (when the number of buckets grows larger
   than the initial number o buckets),
 - make sure that page recycling works correctly.

Because of the last point, it needed large numbers of docs/terms since page
recycling only kicks in on arrays of more than 16KB. However, since then, we
added a MockPageCacheRecycler to track allocation/release of BigArrays and make
sure that all arrays get released, so we can now lower these numbers of docs/
terms to just make sure that array growth is triggered.