Improve IrisInConfigurationSpaceTest.ConvexConfigurationSpace

[jwnimmer-tri]

We noticed in #21029 that the test case IrisInConfigurationSpaceTest.ConvexConfigurationSpace effectively doesn't check anything right now.

(We also had to add a fudge factor for macOS builds to pass CI.)

[RussTedrake]

I don't believe that's accurate. ConvexConfigurationSpace is a good test. In #20974, I added a small additional check to that test in an attempt to cover the new mixing_steps option, and that new aspect of the test was insufficient. But the original test is still good.

[AlexandreAmice]

Yes I did not intend to say that the test does nothing. I meant more specifically that ConvexConfigurationSpace does not effectively check the mixing step option