Fix the min scale e2e test.

[vagababov]

The e2e test for minScale currently checks the target scale by checking spec.scale of the deployment. Which is while a required condition, but it's not enough — we also need to verify that before the revision is marked ready the number of ready endpoints is also at least N.

In what area(s)?

/area autoscale
/area test-and-release
/kind good-first-issue

/cc @markusthoemmes @tanzeeb

[MIBc]

Let me try.

[vagababov]

/assign @MIBc

[tanzeeb]

@MIBc some notes on the previous behaviour here: #5255

There is a race condition in TestMinScale where a deployment can reach
minScale and then scale down before the test has a chance to observe
that it has reached that minScale.

This commit changes the test to observe the desired replicas of the
deployment instead of the actual replicas.

[markusthoemmes]

/good-first-issue

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