Make the test runner of openfisca test more resilient

[bonjourmauko]

A first analysis shows that we are missing tests of the trace option. 😬
And to write these tests we need to find the right syntax; our test command is built on top of pytest and our current tests do not show examples including this pytest syntax change: Node Construction changed to Node.from_parent (which is a little bit more complex than it looks)

Originally posted by @sandcha in #1293 (comment)

See also #1299