Improve doctest output #351
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Description
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Currently the output from doctest isn't very useful, as it only gives a number of how many tests pass and fail at the end, and not a list of which tests fail (see image below). It is also hard to disentangle which output to the terminal is from which tests.

This PR fixes this by providing a custom doctest config, where we pass specific command line arguments to doctest via the method outlined in doctests documentation (see https://github.com/doctest/doctest/blob/master/doc/markdown/main.md )
In the case of the browser tests that xeus-cpp has, this is the only way to pass these command line arguments.
Fixes # (issue)
Type of change
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