Refactor and fix Utf test

[Flamefire]

• Add tests using char16_t/char32_t when available
• Rename test_from/test_to to make C&P issue obvious and fix that.
• Introduce functions for the tests to better group the subtests

Contains the bugfix from #48 and hence closes #48

[codecov[bot]]

Codecov Report

Merging #87 (c79faed) into develop (85f7316) will increase coverage by 0.06%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop      #87      +/-   ##
===========================================
+ Coverage    79.81%   79.88%   +0.06%     
===========================================
  Files           76       76              
  Lines         5911     5931      +20     
===========================================
+ Hits          4718     4738      +20     
  Misses        1193     1193              

Impacted Files	Coverage Δ
test/test_utf.cpp	98.31% <100.00%> (+0.13%)	:arrow_up:
include/boost/locale/utf.hpp	98.22% <0.00%> (+0.07%)	:arrow_up:

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