Improvements for testing with external data

This will be useful once the groundwater flow process outputs something ...

Changes:

- flexible `AddTest()` parameter parsing
- added parameter for
  - executable arguments
  - referenced external data files
  - diff data files
- gives fatal errors when a requirement for a specific wrapper or tester is not met
- added documentation

See [Applications/CLI/Tests.cmake](https://github.com/bilke/ogs/blob/fa36408888cbe5c5475e20c0719b7f132209537c/Applications/CLI/Tests.cmake) for a usage example.
See also a [preliminary documentation](http://docs.opengeosys.org/docs/devguide/testing/test-data).