Add test for wrong on_demand serving

[pedro-psb]

Reproduction for: pulp/pulpcore#5725

Some approaches for solving this:

• Re-design how RemoteArtifact works.
  • Even though that's a big thing, there are some long-standing unresolved issues around this, so we have the motivation.
• On sync, clean RemoteArtifacts that were superseded.
  • That means: if we can tell a remote server has replaced Pkg($nevra_A$, $digest_A$) for Pkg($nevra_A$, $digest_B$), the remote artifacts should reflect that.
  • This more consistent state would have avoided this specific issue, but it feels not so straightfoward to implement.
  • May not make sense if we want to move foward with the broader RA approach.
• On content-app, prioritize remotes associated with the distribution under request
  • As shown here, that's is not a general solution.
  • Again, may not make sense if we want to move foward with the RA re-design approach.
  • As a second plan, that has a low cost for solving only this case and I can't think of any immediate drawbacks.
• On content-app, assure all selected remotes are tried:
  • This feels non-exclusive with others, because that's what I believe it should be doing.
  • On the other hand, it may do "things work" at the cost of taking longer silently. Because we currently can't select the right remote for the general case, it will brute-force until it hopefully works.
  • It's a low-cost approach.