Remove unneccesary parameter In UnitTest's methods

[accmltr]

The src param in the istrue(..) and error_happens(..) methods of the UnitTest class is not neccesary, since the source of the message can automatically specified by the method bodies when calling the actual implementations of these methods on the Testing singelton.

[accmltr]

This would be a breaking change to the API, and should thus be added to the next major version of the plugin in the v2-development branch.

[accmltr]

This issue now falls under the scope of issue #10.

[accmltr]

Issue #10 might take longer than expected to resolve, so this issue will be opened again as an improvement on the interface for the time being.