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
