Add testing of applicable free functions for signed integers

[mborland]

Closes: #170

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.6%. Comparing base (57048b9) to head (6f8c142).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #173     +/-   ##
=========================================
+ Coverage     97.3%   97.6%   +0.4%     
=========================================
  Files          149     166     +17     
  Lines        11286   12670   +1384     
  Branches      4014    4660    +646     
=========================================
+ Hits         10972   12356   +1384     
  Misses         247     247             
  Partials        67      67             

Files with missing lines	Coverage Δ
test/test_signed_abs_diff.cpp	100.0% <100.0%> (ø)
test/test_signed_bounded_charconv.cpp	100.0% <100.0%> (ø)
test/test_signed_bounded_fmt_format.cpp	100.0% <100.0%> (ø)
test/test_signed_bounded_std_format.cpp	100.0% <100.0%> (ø)
test/test_signed_charconv.cpp	100.0% <100.0%> (ø)
test/test_signed_fmt_format.cpp	100.0% <100.0%> (ø)
test/test_signed_gcd.cpp	100.0% <100.0%> (ø)
test/test_signed_lcm.cpp	100.0% <100.0%> (ø)
test/test_signed_limits.cpp	100.0% <100.0%> (ø)
test/test_signed_midpoint.cpp	100.0% <100.0%> (ø)
... and 7 more

... and 2 files with indirect coverage changes

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