Does 3 things:
(1) Use django for modulestore tests
(2) Use normal LMS settings for modulestore tests instead of openedx/tests/settings.py
(3) Simplify some TestCase subclasses by converting them to use ModuleStoreTestCase
Details and rationale:
(1) Currently parts of the modulestore test suite are designed to run "without django", although there is still a lot of django functionality imported at times, and many of the tests do in fact use django. But for the upcoming PR #27565 (moving split's course indexes from MongoDB to MySQL), we will need to always have Django enabled. So this commit paves the way for that change.
(2) The previous tests that did use Django used a special settings file, openedx/tests/settings.py which made some debugging confusing because those tests had quite different django settings than other tests. This change deletes that file and runs the tests using the LMS test settings.
(3) The test suite also contains many different ways of initializing and testing a modulestore, with significant differences in their configuration, and also a lot of repetition. I find this makes understanding, debugging and writing tests more difficult. So this commit also reduces the number of different "test case using modulestore" base classes:
* Simplifies MixedWithOptionsTestCase and MixedSplitTestCase by making them simple subclasses of ModuleStoreTestCase.
* Removes PureModulestoreTestCase.
This should solve the problem of getting `nodejs: not found` when
running `paver test_js_run -s lms` in lms-shell, which cropped up in
the past few days.
I don't know why this stopped working recently, but I don't have `nodejs`
on my recently re-created devstack, and some people with an older devstack
have both. Maybe a recent node.js release got rid of an alias.
Fix the issue that was preventing us from upgrading pytest. pytest does some manipulation of test packages that prevents `pkg_resources` from loading resources from them, but used to contain a workaround for the problem. That workaround was [removed](https://github.com/pytest-dev/pytest/issues/5392) in 4.6.0 as a performance enhancement when pytest switched from `pkg_resources` to `importlib-metadata` for its own entrypoint handling. This tripped up one of our test modules which defined classes that loaded templates from inside a test package. Moving these resources to the parent package fixes the problem.
More and more, `pkg_resources` is being abandoned in favor of `importlib-metadata` and `importlib_resources` as they have a simpler design with much better performance. However, `importlib_resources` doesn't support loading files from any directory which isn't itself a Python package (and doesn't allow direct use of paths including directories within the package). Jinja2 chose a [different approach](https://github.com/pallets/jinja/pull/1082) that we may want to emulate in our resource handling.
Also fixed usage of a removed `pytest.raises()` parameter and a bug in our configuration of the `common/lib` tests that became a problem after the upgrade.
