spec_helper: fix Timeout::Error/SystemExit handling.

[MikeMcQuaid]

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.

Also, because timeouts should be rarer now, reduce the general timeout from 120s to 60s.

add468b is pushed temporarily (and should not be merged) to attempt to reproduce the correct behaviour on a forced timeout This has been removed due to reproduction in https://github.com/Homebrew/brew/pull/9004/checks?check_run_id=1326013554#step:13:38

Fixes #8979

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[MikeMcQuaid]

Confirmed from https://github.com/Homebrew/brew/pull/9004/checks?check_run_id=1326013554#step:13:38 that this is correctly retrying these examples now 🎉