Experimental - DO NOT MERGE: skbase refactor - part 3: test framework

[fkiraly]

Experimental refactor branch.

Not for merge until mature, tests are in place, and based on current skbase.

Moves BaseObject and core base functionality to the skbase package.

Part 3, relies on:

• part 1: skbase refactor - part 1: BaseObject and package dependencies #3151
• part 2: skbase refactor - part 2: all_estimators lookup #3777

Migrated:

• BaseFixtureGenerator, QuickTester, and TestAllObjects, uses skbase.testing under the hood.
• from part 2 - all_estimators - uses skbase.all_objects under the hood now.
• from part 1 - BaseObject. The sktime BaseObject is a child of skbase BaseObject with some added features that are currently sktime specific.

This should be the last step of the refactor/migration.

[fkiraly]

@RNKuhns, I think I've refactored the sktime test framework correctly, but there's now a highly mysterious bug appearing.

The tests seem to be run as expected, but one of the tests - multiprocessing identity (check results are the same with and without multiprocessing) - fails with an odd error, where out of 10 entries in the result a few (four) are by a constant of 1 off.

I cannot explain this with missing excluded tests etc, especially since it's coming from TestAllEstimators rather than TestAllObjects.