Rename nosetests specific mark expression to be more...

This MR generalizes nose-test unit attributes so that pytest adopters can also benefit from the same setup. This should allow a smooth transition from nose to pytest.

@tiago.pereira: please note that expected environment variable name limiting test runs ($NOSE_EVAL_ATTR) was renamed to $TEST_MARK_EXPR so that its usage is clear. I'll rename its usage in bob/nightlies> (scheduled short runs), but I'm not sure this is also used in other packages. Could you please check modify those if required?

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• 750c83fd - [templates] Suggest using pytest instead of nose for test runs

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mentioned in issue bob#267 (closed)