unit tests should cover file content

[jhoblitt]

At present, the unit tests are not checking the content of the generated configuration files. This should be added to catch variable handling errors and ERB template regressions.

[jhoblitt]

@NoodlesNZ Adding unit test coverage of template variable interpolation as you change them may be useful. There's examples of a few different ways of checking file content in here: https://github.com/jhoblitt/puppet-smartd/blob/master/spec/unit/classes/smartd_spec.rb