Strengthen testing of 2017_law.json and TCJA.json reform files

[martinholmer]

This pull requests adds two new tests to the test_reforms.py file. The strengthened testing and the recent change in Policy.LAST_KNOWN_YEAR from 2017 to 2018 (in #2212) revealed the need for some changes in the 2017_law.json and TCJA.json reform files and the need for one small change in the logic that implements a reform containing nonzero values of the _cpi_offset parameter.

cc @MattHJensen @codykallen

[codecov]

Codecov Report

Merging #2217 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #2217   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          12      12           
  Lines        2957    2976   +19     
======================================
+ Hits         2957    2976   +19

Impacted Files	Coverage Δ
taxcalc/parameters.py	100% <100%> (ø)	⬆️
taxcalc/policy.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f3a7947...c97d49f. Read the comment docs.