Add spec for Whitespace check

Now that we have a convenient way to test anything involving
`StagedFile`s, we should try to add tests for these files.

Change-Id: If1c6cfee179cbd2dd13f2ed450b04f23cbe19195
Reviewed-on: https://gerrit.causes.com/25161
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>