Fix id tool tests
Some tests were added to check warnings, which were not working in
Python 2 because the warning message was shown just once.
This is fixed by adding a warnings.simplefilter("always") inside
the catch_warnings context manager, as recommended in
https://docs.python.org/3/library/warnings.html#testing-warnings .
Note that the previous filter is restored on leaving the context
manager.
Tests pass locally and a test suite is in progress.