This commit makes the job controller re-honor exponential backoff for
failed pods. Before this commit, the controller created pods without any
backoff. This is a regression because the controller used to
create pods with an exponential backoff delay before (10s, 20s, 40s ...).

The issue occurs only when the JobTrackingWithFinalizers feature is
enabled (which is enabled by default right now). With this feature, we
get an extra pod update event when the finalizer of a failed pod is
removed.

Note that the pod failure detection and new pod creation happen in the
same reconcile loop so the 2nd pod is created immediately after the 1st
pod fails. The backoff is only applied on 2nd pod failure, which means
that the 3rd pod created 10s after the 2nd pod, 4th pod is created 20s
after the 3rd pod and so on.

This commit fixes a few bugs:

1. Right now, each time `uncounted != nil` and the job does not see a
_new_ failure, `forget` is set to true and the job is removed from the
queue. Which means that this condition is also triggered each time the
finalizer for a failed pod is removed and `NumRequeues` is reset, which
results in a backoff of 0s.

2. Updates `updatePod` to only apply backoff when we see a particular
pod failed for the first time. This is necessary to ensure that the
controller does not apply backoff when it sees a pod update event
for finalizer removal of a failed pod.

3. If `JobsReadyPods` feature is enabled and backoff is 0s, the job is
now enqueued after `podUpdateBatchPeriod` seconds, instead of 0s. The
unit test for this check also had a few bugs:
    - `DefaultJobBackOff` is overwritten to 0 in certain unit tests,
    which meant that `DefaultJobBackOff` was considered to be 0,
    effectively not running any meaningful checks.
    - `JobsReadyPods` was not enabled for test cases that ran tests
    which required the feature gate to be enabled.
    - The check for expected and actual backoff had incorrect
    calculations.