Add coverage of to_chars fallback path

[mborland]

This path is only taken by extreme values on platforms with 128-bit long doubles so force direct testing.

[codecov]

Codecov Report

Merging #106 (516601b) into develop (f55167d) will increase coverage by 0.67%.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #106      +/-   ##
===========================================
+ Coverage    75.89%   76.56%   +0.67%     
===========================================
  Files           28       28              
  Lines         3957     3955       -2     
===========================================
+ Hits          3003     3028      +25     
+ Misses         954      927      -27     

Files	Coverage Δ
include/boost/charconv/to_chars.hpp	88.42% <100.00%> (+13.94%)	⬆️

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f55167d...516601b. Read the comment docs.