Refactor the `regular_tax_before_credits` variable and add unit tests

It currently includes a number of variables which are not descriptive as well as calculation steps which do not seem transparent

First step would be to compare the calculation to `capital_gains_tax` (up until the inclusion of `income_tax_main_rates`) and to identify which steps can be consolidated for clarity

Secondly, unit tests should be added to test the file - it will be not done in #4815

We should also add references and tie the hard coded `0.28` parameter from the `dwks41` calculation to the amt bracket parameter which is refactored in #4815