Rename `RegressionMixin` to `RegressionTestPlugin` or similar

[vkarak]

The RegressionMixin is used for extending the functionality of existing tests, thus the more accurate name of "plugin".

The plan is for 4.9 to deprecate the RegressionMixin name in favor of the new name and complete remove it in 5.0

[jack-morrison]

RegressionMixin inherit from RegressiontestPlugin and issue deprecation warning.