(Smoke) testing

[kba]

To ensure code remains functional, some basic test of functionality is required. Helps while refactoring the code (e.g. #28 #9 #20)

First, fix output (currently XML declaration is duplicated for each page tree).

Then have an example (e.g. the one in @OCR-D/spec) with the expected output and create a test script to ensure it's still produced.

Make sure the tool is deployable and functional by adding CI to test continuously. Extend test script / examples to reflect extended CLI/specs.

[kba]

Similar UB-Mannheim/ocr-fileformat#41

[kba]

PoC in https://github.com/kba/pyocrd/tree/minimal-test