Fixed #21430 -- Added a RuntimeWarning when unpickling Models and QuerySets from a different Django version.

Thanks FunkyBob for the suggestion, prasoon2211 for the initial patch,
and akaariai, loic, and charettes for helping in shaping the patch.
Fixed #22068 -- Made runtests.py remove trailing slashes from test labels.

When using tab-completion it's easy to accidentally run a test with
a trailing slash, which causes INSTALLED_APPS to be set incorrectly.
Normalize the test labels to avoid this common error.

Thanks MattBlack for the suggestion.
Created a new tests folder (`model_options`).

And moved `tablespaces` option tests to it.
The new folder can be used to test models/options, like the new option
added in refs #22778.