Feature/ordered tests

## What This Does

Allows for ordered test suites.

- _testsToRun is now an NSArray rather than an NSSet
- adds NSArray versions of runTests:withCompletionBlock: and testsToRun:withFocus:
- changes the names of the original NSSet versions of the two above methods to explicitly state that they use sets and update all references to them.

This is a super basic implementation and I touched as little code that wasn't touching the new array methods as possible.

After this is merged, some changes will need to be made to our feature/subliminal branch to make it run with these changes (just waiting for this to make those commits).

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@j-mutter