formula_installer: don't always install test deps.
formula_installer: don't always install test deps.

Fixes #6470.

MikeMcQuaid/brew,vitorgalvao/brew,reitermarkus/brew,nandub/brew,sjackman/homebrew,EricFromCanada/brew,claui/brew,claui/brew,EricFromCanada/brew,sjackman/homebrew,mahori/brew,EricFromCanada/brew,nandub/brew,MikeMcQuaid/brew,vitorgalvao/brew,JCount/brew,EricFromCanada/brew,mahori/brew,sjackman/homebrew,mahori/brew,Homebrew/brew,claui/brew,vitorgalvao/brew,reitermarkus/brew,reitermarkus/brew,nandub/brew,JCount/brew,konqui/brew,konqui/brew,JCount/brew,MikeMcQuaid/brew,sjackman/homebrew,konqui/brew,reitermarkus/brew,mahori/brew,Homebrew/brew,Homebrew/brew,nandub/brew,konqui/brew,Homebrew/brew,claui/brew,MikeMcQuaid/brew,vitorgalvao/brew,JCount/brew