Fix compiler warnings when running mix test

[devonestes]

When we ran mix test we were getting several compiler warnings. Here's the
ouput that we got before this PR:

warning: behaviour Set undefined
  exercises/custom-set/example.exs:1

warning: the Inspect protocol has already been consolidated, an implementation for CustomSet has no effect
  exercises/custom-set/example.exs:126

warning: function digits/1 is private, @doc's are always discarded for private functions
  exercises/largest-series-product/example.exs:3

warning: function slices/2 is private, @doc's are always discarded for private functions
  exercises/largest-series-product/example.exs:17

warning: redefining module DNA (current version defined in memory)
  exercises/nucleotide-count/example.exs:1

warning: redefining module DNA (current version defined in memory)
  exercises/rna-transcription/example.exs:1

Including tags: [:pending]

warning: redefining module ChangeTest (current version defined in memory)
  exercises/flatten-array/flatten_array_test.exs:8

warning: redefining module ChangeTest (current version defined in memory)
  exercises/hexadecimal/hexadecimal_test.exs:8

warning: redefining module DNATest (current version defined in memory)
  exercises/nucleotide-count/nucleotide_count_test.exs:8

warning: redefining module DNATest (current version defined in memory)
  exercises/rna-transcription/rna_transcription_test.exs:8

warning: the Inspect protocol has already been consolidated, an implementation for BinTree has no effect
  exercises/zipper/zipper_test.exs:16

I addressed all of the warnings except those for CustomSet since those
warnings are taken care of in #226. Many of them were naming issues for
modules, and funny enough this fix of the compiler warnings pointed out that we
had a typo in a test that should have failed but wasn't failing! I've also
fixed that typo.

[parkerl]

Awesome!!! 🎉