Backport of scheduler: tolerate having only one dynamic port available into release/1.5.x #17623
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Backport
This PR is auto-generated from #17619 to be assessed for backporting due to the inclusion of the label backport/1.5.x.
The below text is copied from the body of the original PR.
If the dynamic port range for a node is set so that the min is equal to the max, there's only one port available and this passes config validation. But the scheduler panics when it tries to pick a random port. Only add the randomness when there's more than one to pick from.
Adds a test for the behavior but also adjusts the commentary on a couple of the existing tests that made it seem like this case was already covered if you didn't look too closely.
Fixes: #17585