Refactor the regular_tax_before_credits variable and add unit tests

[Mshuning]

Fixes #4817

[PavelMakarchuk]

This is a start, can you add the relevant tax form lines, especially for the ambiguous variables names
e.g.

# Line 17
dwks19 = ... 

[Mshuning]

2017 Instructions for Schedule D

2023 Instructions for Schedule D

@PavelMakarchuk The differences between the 2017 form and the 2023 form are from step 19 in 2017 to steps 19-21 in 2023.

[Mshuning]

https://www.irs.gov/pub/irs-prior/i1040sd--2017.pdf#page=15
Line 26 says:

26. Add lines 19 and 20.

https://github.com/PolicyEngine/policyengine-us/blob/master/policyengine_us/variables/gov/irs/tax/federal_income/alternative_minimum_tax/regular_tax_before_credits.py
While the old code:

dwks26 = min_(dwks19, dwks20)