tests: Migrate plugin factory test to internals directory

tests/internals/pluginfactory.py

tests/internals/pluginfactory/anotherelement/__init__.py

tests/internals/pluginfactory/anotherelement/foo.py

tests/internals/pluginfactory/anothersource/__init__.py

tests/internals/pluginfactory/anothersource/foo.py

tests/internals/pluginfactory/badsetup/__init__.py

tests/internals/pluginfactory/badsetup/foo.py

tests/internals/pluginfactory/badversionelement/__init__.py

tests/internals/pluginfactory/badversionelement/foo.py

tests/internals/pluginfactory/badversionsource/__init__.py

tests/internals/pluginfactory/badversionsource/foo.py

tests/internals/pluginfactory/customelement/__init__.py

tests/internals/pluginfactory/customelement/foo.py

tests/internals/pluginfactory/customsource/__init__.py

tests/internals/pluginfactory/customsource/foo.py

tests/internals/pluginfactory/nosetup/__init__.py

tests/internals/pluginfactory/nosetup/foo.py

tests/internals/pluginfactory/notatype/__init__.py

tests/internals/pluginfactory/notatype/foo.py

tests/internals/pluginfactory/wrongtype/__init__.py