Replace gaudi_add_tests(pytest...) with gaudi_add_pytest(...)

Marco Clemencicrequested to merge
better-use-of-pytest into master

gaudi_add_tests(pytest ...) is deprecated in favour of gaudi_add_pytest(...) and this MR makes that change.

I also modified the tests to work better when invoked by pytest, mostly to sidestep gaudi/Gaudi#354 (closed).

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