Fixed flake8 errors.
Removed old test runner test cases.

These files were added as part of #12658, which pertained to the old django.test.simple test runner. No tests call them anymore.
Fixed #22402 -- Consolidated model_inheritance tests.

The model_inheritance_same_model_name tests couldn't be run without the
model_inheritance tests. Make the problem go away by merging them.

Thanks timo for the report.
Further consolidated the model_inheritance tests.
Consolidated get_or_create tests.
Consolidated expressions tests.
Appeased flake8 2.1.0.
Fixed monkeypatching in a staticfiles test.
Fixed migrate when called inside a transaction.

This is useful for tests manually calling migrate inside a testcase,
for normal usage this should make no difference, since there is no
surrounding transaction after all. If there is one we still try to
leave the transaction in a useable state (for postgres at least).

If this commit turns out to be causing issues, settings savepoint=False
is probably the right fix :)
Merge pull request #2592 from prestontimmons/remove-test-runner-cases

Removed old test runner test cases.
Corrected the section identifier for MySQL unicode reference.
Fixed adding new GIS column in PostGIS 1.x migration

Refs #22451.
Fixed removal of GIS column in PostGIS 1.x migration

Refs #22481.