Global sleep function for testing

[pda]

WithSleepFunc is a great way to test retries without waiting for them, and allows assertions against the requested sleep time 👌🏼

However, threading this through from tests to the retryer can be tricky.
I'd love to be able to set it globally for the duration of a test like this:

func TestRetryingThings(t *testing.T) {
  oldSleepFunc := roko.GlobalSleepFunc
  roko.GlobalSleepFunc = func(t time.Duration) {}
  defer func() { roko.GlobalSleepFunc = nil }()

  result := doThingsThatIndirectlyRetry()
  // assert things
}

Or maybe it can be designed to be defer-friendly nested/reentrant-ish:

func TestRetryingThings(t *testing.T) {
  revert := roko.WithGlobalSleepFunc(func (t time.Duration) {})
  defer revert() // restore whatever the previous global sleep func was

  result := doThingsThatIndirectlyRetry()
  // assert things
}