Bring MeanImputer in line with new testing setup

[limlam96]

What?

There are a few required changes for this, summarised below from (#150)

• Double check "minimal attribute dict" fixture has the correct set of minimal arguments required
• Set up the TestInit TestFit, TestTransform, and TestOtherBaseBehaviour classes to inherit the appropriate test classes and then copy the setup_class class method into each test class, changing transformer_name to the transformer you are testing (examples linked) .
• Extra tests specific to this class should be added, e.g. if this transformer also needed to check that when len(mappings)>2 then mappings is an ordered dict, you would add this to TestInit. Where extra tests would apply to all child classes, they should instead be added to the parent test class.
• Delete any unnecessary tests from original transformer test file (i.e. tests testing inherited behaviour that is now tested by the inherited test classes). An example would be. In general asserting calls is a red flag, as this tests the applied process rather than the desired outcome.
• Debug the failing tests from the base test suite.
• Bring the test file in line with best practices by insuring there are no implementation tests (in this case looks like these will have been removed as part of the base test changes)
• Bonus: If you identify any gaps in the testing, please set up new issues :)

Why?
Revamp tubular testing to make package more maintainable/modifiable

[limlam96]

There are some changes here here to the BaseImputer tests that will be useful to this, if this PR is not complete at time of picking up then would branch off of it!