Add directions to CONTRIBUTING for common mistake when running tests
Fix i18n for boolean filter (also fixes #2170)

Similar to commit 47fe4c5 but fixes regression that caused untranslated
labels for boolean filters.
Merge pull request #2221 from dmfrancisco/pr-fix-boolean-filter-i18n

Fix i18n for boolean filter (updates the fix for #2170)
improve test perf by deferring garbage collection

Our tests run noticeably faster by only running the garbage collector every 15 seconds.
Inspired by: ariejan.net/2011/09/24/rspec-speed-up-by-tweaking-ruby-garbage-collection