testInvalidationBug: Tolerate multiple indexation activities being spawned.
@jm : Please check if these changes are compatible with the original intent of these tests.
In a future patchset I need tests to work with fulltext indexation enabled, which caused test failures where the test code was checking exactly how many activities were spawned. Fulltext indexation spawning an extra activity of course breaks such assumption, and I believe tests should not be sensitive to such extra activity. Hence these changes. I already pushed several more in the same idea, but in tests not as critical, and not going as deep into the details of how zope behaves, so I want to make sure I am not denaturing the intent of these tests.