TST: stats: Move genexpon from xslow to slow fit test sets.

[WarrenWeckesser]

This is a follow-up to #17708. The time required to fit genexpon is now on the same order as many of the other distributions in the "slow" lists, so I've moved it there from the "xslow" lists.

I also alphabetized the lists that are changed, which makes it much easier to read the lists and check if a distribution is in there.

[mdhaber]

This seems to do what it intends to do and doesn't make any unexpected changes. Also, I didn't see this test added to the list of the ten slowest in any jobs I checked. Let's let the last job finish add then merge.