test-bot: prevent infinite loop caused by unavailable formula.
test-bot: prevent infinite loop caused by unavailable formula.

Noted, `brew tap foo` will not fail if the tap is already installed.

mgrimes/brew,maxim-belkin/brew,DomT4/brew,EricFromCanada/brew,palxex/brew,amar-laksh/brew_sudo,amar-laksh/brew_sudo,pseudocody/brew,AnastasiaSulyagina/brew,bfontaine/brew,mgrimes/brew,claui/brew,nandub/brew,mgrimes/brew,Linuxbrew/brew,MikeMcQuaid/brew,hanxue/linuxbrew,EricFromCanada/brew,aw1621107/brew,ilovezfs/brew,JCount/brew,palxex/brew,zmwangx/brew,zmwangx/brew,Linuxbrew/brew,Linuxbrew/brew,rwhogg/brew,gordonmcshane/homebrew,ilovezfs/brew,sjackman/homebrew,alyssais/brew,hanxue/linuxbrew,MikeMcQuaid/brew,AnastasiaSulyagina/brew,Homebrew/brew,claui/brew,reelsense/brew,bfontaine/brew,maxim-belkin/brew,muellermartin/dist,aw1621107/brew,mahori/brew,tonyg/homebrew,muellermartin/dist,konqui/brew,vitorgalvao/brew,sjackman/homebrew,gordonmcshane/homebrew,hanxue/linuxbrew,staticfloat/brew,Homebrew/brew,pseudocody/brew,jmsundar/brew,konqui/brew,mahori/brew,rwhogg/brew,DomT4/brew,JCount/brew,Homebrew/brew,mahori/brew,maxim-belkin/brew,gregory-nisbet/brew,reelsense/brew,MikeMcQuaid/brew,tonyg/homebrew,Linuxbrew/brew,staticfloat/brew,reitermarkus/brew,mahori/brew,muellermartin/dist,AnastasiaSulyagina/brew,claui/brew,Homebrew/brew,reitermarkus/brew,EricFromCanada/brew,JCount/brew,MikeMcQuaid/brew,konqui/brew,toonetown/homebrew,amar-laksh/brew_sudo,jmsundar/brew,sjackman/homebrew,alyssais/brew,gregory-nisbet/brew,EricFromCanada/brew,vitorgalvao/brew,vitorgalvao/brew,palxex/brew,gregory-nisbet/brew,staticfloat/brew,JCount/brew,rwhogg/brew,zmwangx/brew,bfontaine/brew,sjackman/homebrew,toonetown/homebrew,pseudocody/brew,nandub/brew,jmsundar/brew,reelsense/brew,reitermarkus/brew,gordonmcshane/homebrew,toonetown/homebrew,konqui/brew,nandub/brew,reitermarkus/brew,tonyg/homebrew,DomT4/brew,ilovezfs/brew,nandub/brew,claui/brew,aw1621107/brew,vitorgalvao/brew,alyssais/brew,DomT4/brew