BuildPulse is trying to find flaky tests for us but, given the previous
model of using `rspec-retry`, it would rarely find them. Instead, let's
try to always rerun `brew tests` multiple times, report to BuildPulse
each time (by moving the reporting logic into `brew tests`) and disable
`rspec-retry` when using BuildPulse.
While we're here, let's enable `rspec-retry` locally so we don't have
flaky tests biting maintainers/contributors there.