[test] fix ngrams test by having a better non-empty terms generator

Ref #488
4 jobs for 471-dev-node-multiterms in 46 minutes and 4 seconds (queued for 6 seconds)
latest
Status Job ID Name Coverage
  Cabal
passed #18921
cabal

00:20:03

 
  Stack
passed #18922
stack

00:03:51

 
  Bench
manual #18923
allowed to fail manual
bench
 
  Test
passed #18924
test

00:22:09