Improve test helper docs #1992

Job	Run time
javascript_test	1m 28s
mongodb_test (gemfiles/mongoid_6.gemfile)	1m 32s
lint	1m 4s
mongodb_test (gemfiles/mongoid_7.gemfile)	1m 26s
system_tests	1m 57s
postgres_test	2m 16s
test (Gemfile, 3.2)	1m 7s
test (gemfiles/rails_7.0.gemfile, 3.1)	1m 25s
test (gemfiles/rails_master.gemfile, 3.1)	1m 49s
test (gemfiles/rails_7.0.gemfile, 3.2)	1m 26s
test (gemfiles/rails_7.1.gemfile, 3.3)	1m 33s
test (gemfiles/rails_master.gemfile, 3.3)	1m 49s
	18m 52s

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