Add testing mode

[ccremer]

Writing templates can be tedious with lots of try-and-error
It should be possible to render templates in a local non-Git directory and compare them with a given set of expected file contents.

With that, it's possible to write kind of "unit tests" of a template.

Proposed file structure:

.tests
└── case-1
    └── README.adoc
tests
├── case-1
│   ├── README.adoc
│   └── .sync.yaml
└── tests.yaml

Test cases are put into arbitrary-named folders inside the tests directory in a template repository.
Each of these folders all the files that are expected to be rendered.
The .sync.yaml contains the template variables with the exact same syntax as .sync.yaml for a managed repository (in fact it's treated as a repository).
The .tests folder exists only after running gsync test and contains the rendered files based on the files in tests with the same directory structure.
The file tests/tests.yaml may contain any global variables (similar to config_defaults.yaml).
That way, the render process is almost exactly the same.

[ccremer]

Implemented in #171