Improve `SetMapRulesTest`, based on 2.13.3 experience

[SethTisue]

splitting this off from as a post-release to-do item, from scala/scala#9078

[SethTisue]

@lrytz I'm finally looking at this, after a long delay...

you wrote:

Add a generic test case to SetMapRulesTest that would have caught the bug before we found it in scala-collections-laws.

now that I'm looking at this again, I guess I don't understand why this is necessary (or worth the effort). we have scala-collection-laws, and we always run it before release. in principle, anything in scala-collection-laws could also be duplicated in scala/scala. I don't see how doing that piecemeal makes sense.

admittedly it would be nice if we had some more convenient, automatic way to run scala-collection-laws on a PR besides manually queueing up a Jenkins run. we have a ticket on it, even: #411