test-bot: fix diff_formulae
test-bot: fix diff_formulae

* Fix regression caused by 05649cee60e5349afc328070a0c0e4f6835f97f0
* Use `Tap#formula_file?` to filter formula.

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