brew install fschloesser/test/<formula>
Or brew tap fschloesser/test and then brew install <formula>.
This could be brew install fschloesser/test/test
Or install via URL (which will not receive updates):
brew install https://raw.githubusercontent.com/fschloesser/homebrew-test/master/Formula/<formula>.rb
brew help, man brew or check Homebrew's documentation.