Broaden scope of test_taxcalcio.py to increase code coverage

[martinholmer]

Part of a series including #1225 and #1228 and #1232.

[codecov-io]

Codecov Report

Merging #1234 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1234   +/-   ##
======================================
  Coverage    99.2%   99.2%           
======================================
  Files          38      38           
  Lines        2650    2650           
======================================
  Hits         2629    2629           
  Misses         21      21

Impacted Files	Coverage Δ
taxcalc/taxcalcio.py	99.37% <ø> (ø)	✅

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