spec_helper: fix Timeout::Error/SystemExit handling.
spec_helper: fix Timeout::Error/SystemExit handling.

These need to be manually caught and set otherwise they will not be
retried by `rspec-retry`. This is particularly annoying and a cause of
CI failures when tests timeout but are not retried.

Fixes https://github.com/Homebrew/brew/issues/8979

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