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.