Fix incorrect parsing of fixed-length second fractions

[chris-leach]

When parsing, the "fixed-length decimal fraction" specifiers (%.3f, %.6f, %.9f) are currently treated identically to the variable-length specifier (%.f).

This causes issues when the fractional part is immediately followed by more digits signifying something completely different (you might think this bizarre, and I would agree, but I've seen upsetting things in vendor-specific log formats).

This PR changes the parser for those specifiers to consume only the exact number of digits specified, returning TooShort if there aren't enough. Test cases added for all three.

Fixes #1592.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.60%. Comparing base (3b4af71) to head (b6defb0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1705      +/-   ##
==========================================
+ Coverage   90.59%   90.60%   +0.01%     
==========================================
  Files          38       38              
  Lines       16028    16058      +30     
==========================================
+ Hits        14520    14550      +30     
  Misses       1508     1508              

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[djc]

Thanks!