support setting_changed test signal for settings.DJPT_KNOWN_LIMITS_DOTTED_PATHS

[zsoldosp]

as currently it's initialized once

the difficulty why it wasn't done as part of #4 is that

• there are many different registries created during testing and they all might be effected. Some might mock out the actual class loading (as good unit tests should), but some might not, and thus it would cause issues when the mocked tests change it to ['blah', 'no.such.module']
• also, this could get even more complicated after use the registry for setting up the collectors with id_ #11