Unittests for MADescriptionProvider

[Assargadon]

PR #131 introduces MADescriptionProvider, but did not provide any specific unit-tests for it. The provider is used in other tests, so to some extent it's tested...somewhat.

However, separate unit tests just for it seems to be useful.

P.S. It is Especially worth testing the scenario when non-MAContainer element is provided. I may imagine, say, MASingleOption with some scalar type to be described and then re-used in several places. Or something like this.