Skipped sleep in VCR replaying mode

[modular-magician]

In SleepInSecondsForTest(), skip the sleep when VCR_MODE is REPLAYING. This makes the assumption that the sleep is always related to a condition that's already mitigated by the fact that we're using prerecorded fixtures, but it could be extended later to override this behavior if necessary.

Fixes #20916

Replaying mode with change:

--- PASS: TestAccResourceGoogleProjectDefaultServiceAccountsDisable (24.32s)
PASS
ok  	github.com/hashicorp/terraform-provider-google/google/services/resourcemanager	25.464s

without change (this one has a 5s sleep, so the difference is small, but you can see it more dramatically on something with a longer sleep)

--- PASS: TestAccResourceGoogleProjectDefaultServiceAccountsDisable (29.51s)
PASS
ok  	github.com/hashicorp/terraform-provider-google/google/services/resourcemanager	30.720s

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Derived from GoogleCloudPlatform/magic-modules#12785