Fixed minor warnings in tests.
Fixed #19543 -- implemented SimpleLazyObject.__repr__

Thanks to Florian Hahn for the patch
PEP8 cleanup of functional.py
Merge pull request #882 from loic/testfix

Fixed minor warnings in tests.
Fixed #19951 -- Admin: Invalid model PK values shouldn't cause 500 status.

Avoid ValueError in admin when passing a string as PK for an inherited
model with an integer PK field.

Thanks ikks for the patch. See also #11191.
Fixed --ignorenonexistent for XML based fixtures.

Ticket #19998