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With this change both C++ and Python write to sys.stdout which resolves
the capture issues noted in #351. Therefore, the related workarounds are
removed.

pytest can capture test output both globally (controlled by the cmd line
flag --capture) or locally (`capsys` and `capfd` fixtures). Enabling both
methods at the same time causes problems on Windows: test output is not
captured sometimes, resulting in test failure. This happens seemingly at
random.

This workaround disables global output capture ("-s", i.e. "--capture=no")
leaving only the local capture fixtures. As a side-effect test output on
AppVeyor CI is a little messy, but this will have to do until a better
solution is found.

Use simple asserts and pytest's powerful introspection to make testing
simpler. This merges the old .py/.ref file pairs into simple .py files
where the expected values are right next to the code being tested.

This commit does not touch the C++ part of the code and replicates the
Python tests exactly like the old .ref-file-based approach.