tests: improve "Output is not processed with visible popup menu" (#1858)
tests: improve "Output is not processed with visible popup menu" (#1858)

This was flaky on vim8069 (https://circleci.com/gh/neomake/neomake/6444).
neomake/neomake,nerdrew/neomake,codeinabox/neomake,nerdrew/neomake,neomake/neomake