This is a follow up to #4719. It appears that singularize and pluralize
are supposed to be idempotent - i.e. when you call singularize or
pluralize multiple times on the same string, you should get the same
result. (At least for the "officially supported" cases that the stock
inflector is designed to handle.) #4719 added the missing tests for
regular cases, and this commit added the missing tests for the
irregularities.

While I'm at that, I also synced up the irregularity test cases with
the current set of irregularity cases that we ship out-of-the-box.