PEP8 cleanups for some unit test modules
Add some functions to Python unit test utilities
Update label tests to ensure setup happens only once and load/delete only layer to test
Refactor composer label test to work with svg, pdf and multi-resolution classes

- Initial (untested) svg and pdf render-to-image setup
Convert label unit tests to using only QgsMapSettings

- Rebuild all control images
Condense labeling test class inheritance and set up local server params from map settings
Add allowable mismatch to debug output for render checker
Add allowable mismatch per group/test for labeling tests