keychain: fix url, license, test (#67281)
keychain: fix url, license, test (#67281)

Main URL is defunct, mirror still works, but the home page says:
> The latest release of Keychain can be found on the keychain GitHub release page.

The GitHub tarball has more content, but none of it is used here, and the installed stuff is byte-identical with the originals, so no revision bump is needed.

Also added `license` and a proper test.
chrisfinazzo/homebrew-core,lemzwerg/homebrew-core,Homebrew/homebrew-core,mfikes/homebrew-core,chrisfinazzo/homebrew-core,cblecker/homebrew-core,nandub/homebrew-core,uyjulian/homebrew-core,zyedidia/homebrew-core,dsXLII/homebrew-core,Moisan/homebrew-core,nbari/homebrew-core,stgraber/homebrew-core,zyedidia/homebrew-core,lemzwerg/homebrew-core,Moisan/homebrew-core,mfikes/homebrew-core,makigumo/homebrew-core,nbari/homebrew-core,mbcoguno/homebrew-core,JCount/homebrew-core,uyjulian/homebrew-core,filcab/homebrew-core,JCount/homebrew-core,makigumo/homebrew-core,adam-moss/homebrew-core,stgraber/homebrew-core,Linuxbrew/homebrew-core,sjackman/homebrew-core,dsXLII/homebrew-core,sjackman/homebrew-core,spaam/homebrew-core,mbcoguno/homebrew-core,nandub/homebrew-core,mbcoguno/homebrew-core,cblecker/homebrew-core,Linuxbrew/homebrew-core,spaam/homebrew-core,adam-moss/homebrew-core,Homebrew/homebrew-core,filcab/homebrew-core