test: cover pct_string_view::decode

[alandefreitas]

This commit simplifies duplicated code so that we tests all paths for the pct_string_view::decode function. Unreachable paths are also marked.

This is a partial solution to #828, where src/detail/decode.cpp has low coverage.

[cppalliance-bot]

GCOVR code coverage report https://835.url.prtest2.cppalliance.org/gcovr/index.html
LCOV code coverage report https://835.url.prtest2.cppalliance.org/genhtml/index.html Coverage Diff https://835.url.prtest2.cppalliance.org/gcovr/coverage_diff.txt

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.13%. Comparing base (9d4a4a5) to head (0ee988f).

Additional details and impacted files

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