Bring MeanResponseTransformer in line with new testing setup

What?

There are a few required changes for this, summarised below from (https://github.com/lvgig/tubular/issues/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. In this case TestInit will need an additional check on e.g the arg prior.
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 remaining implementation tests
Bonus: If you identify any gaps in the testing, please set up new issues :)

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

lvgig / tubular