Attempt to improve nqjoin test

[mattdowle]

Attempt to reduce resources needed due to fail in (unrelated) PR #2719.

 Running test id 1648.003      
 Running test id 1648.004      Error: memory exhausted (limit reached?)
 Error: memory exhausted (limit reached?)

Fairly minor improvements to speed/efficiency in the nqjoin tests 1641-1652. Steps taken towards simplification which might reduce many small objects a little bit; e.g. to avoid one level of lapply and a Reduce instead of lapply(,<<-)
The gc() is attempt to free heap a bit earlier once per test number.

[codecov-io]

Codecov Report

Merging #2732 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2732   +/-   ##
=======================================
  Coverage   93.41%   93.41%           
=======================================
  Files          61       61           
  Lines       12281    12281           
=======================================
  Hits        11472    11472           
  Misses        809      809

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