There seems to be an additional test that gets run on submit that doesn't exist in the test cases. It fails: "Expected: false, instead got: true" and I have no idea why.

Probably... though it should probably actually be something like "1 should not be plural". But after a certain point (number of solutions, maybe?) CW locks the tests so the author can't invalidate a ton of previously-accepted solutions. Thanks for the suggestion, though.

This test seems incorrect:

Test.assertEquals(plural(1), false, "Plural for 1");

Shouldn't it be:

Test.assertEquals(plural(1), false, " Not Plural for 1");

It doesn't affect the outcome since your just testing for true or false, but it might lead to some confusion