test: add test to check if a format string is plain

[Viatorus]

No description provided.

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (a583ffa) 99.40% compared to head (c59f50f) 99.42%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #65      +/-   ##
==========================================
+ Coverage   99.40%   99.42%   +0.01%     
==========================================
  Files          56       56              
  Lines        5866     5877      +11     
==========================================
+ Hits         5831     5843      +12     
+ Misses         35       34       -1     

Files	Coverage Δ
include/emio/buffer.hpp	99.35% <100.00%> (ø)
test/unit_test/detail/test_dragon.cpp	100.00% <ø> (ø)
test/unit_test/test_buffer.cpp	100.00% <100.00%> (ø)
test/unit_test/test_format_string.cpp	100.00% <100.00%> (ø)
test/unit_test/test_writer.cpp	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[Viatorus]

ICE with clang-15...