Originally the test would check for the index of a title in an array of
unique challenge titles. However, the index of the title within the
array isn't important for this test, so I simplified the code using
@Bouncey's suggestion in PR #17035.
Also changed grammar for the error that's thrown when a challenge
title isn't a valid string.
BREAKING CHANGE: None
Due to the recent problem with two challenges having the same title, I
added a check for unique titles to the test suite so that hopefully
won't happen again.
Addresses a comment in issue #16906, which was recently closed.
BREAKING CHANGE: None
Closes#16906