Create indentation tests

[bbatsov]

Such tests would be really useful and will make the code less vulnerable to regressions.

We can borrow ideas from elixir-mode and swift-mode.

[andrewhr]

@bbatsov looks interesting. One idea though: since indentation is idempotent, the test macro could receive only the expected version. That way the tests could read a bit more like documentation. Something like:

(def-indentation-test vertically-aligns-function-args-spanning-multiple-lines 
  "
(filter even?
        (range 1 10))
")

Yep, I stealed the example from your style guide 😉

[the-kenny]

You could even read the forms from an external file - no need to embed code in strings which always seems messy.