Use indexOf for string matching.  Why can't we test tinytest?

We noticed that the current implementation of the include assertion method for strings just checks for matching of one letter.  This is sub-optimal as I don't see any actual string matching case where you care about 1 letter.

Instead let's use indexOf and test that it is greater than -1, which only returns -1 if we don't find a match of a substring within a string.  Anything greater than -1 means we found a substring match in the string.

/cc @ryw