Bring ArbitraryImputer in line with new testing setup

[limlam96]

What?

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

• Update "minimal attribute dict" fixture in conftest.py with the correct set of minimal arguments required
• Set up the TestInit, TestFit, TestTransform, and TestOtherBaseBehaviour methods to inherit the appropriate test classes (see an example Updated Mapping Transformer Tests) and then copy the setup_class class method into each test class, changing transformer_name to the transformer you are testing.
• Delete any unnecessary tests from original transformer test file (i.e. tests testing inherited behaviour that is now tested by the inherited test classes).
• 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]

Going to pick this one up myself to test the process

[limlam96]

completed here