Extend permute_identifier for more unique combinations

[timrcm]

Extend permute_identifier to replace last two characters for more unique combinations

[hartmans]

There's a general design principle called the rule of zero, one or infinity. Briefly, either you don't need to do something at all, it's a singleton, or it's worth taking the time to build in extensibility rather than arbitrary limits.
It doesn't always apply, but I think it applies here.
By the time you overflow one character, I think it's time to come up with a better design.
What I would probably do is add one character if the identifier is not already 14 chars long.
Then from the last character, modify backward until you find a unique combination.
I would probably assume that only one character is going to be modified from the original identifier.
So progression would be something like

• add if there's room.
• Go through increments for added character.
• walk back through identifier incrementing/modifying one character at a time
• yielding until you find something unique at each step.

While you are in there, make a change in a separate commit to get the tests passing.

[timrcm]

That is a great design point that I will keep in mind, thanks!

I'll take a look at what is going on with the tests separately.