allowing negative values in fixed buffer distance
[labeling] Make test font available for app, tests, server without requiring font install

- Load test font from testdata.qrc if app or server built with ENABLE_TESTS
- Add global loading of test font in utilities.py
[labeling] Update unit test control images, features db and .qml styles
Add simple local test server convenience classes for unit tests
[labeling] Add local preconfigured test server unit tests class

- Add control images for server output
- Add class for comparison against canvas output