Don't wrap diagnostic warnings

[ralismark]

The print_ln() calls in the original code would also output newlines to the current diagnostic. Changing it to directly print newlines fixes this issue (though interestingly, the code for the SELECTOR_TERM_AND_LOG already did this). This change shouldn't affect the contents of the tex log, only the nicer-looking warnings that get printed to the terminal.

Fixes #684, I think, though that issue might be referring to the actual tex log that's being printed to rust_stdout.

[codecov]

Codecov Report

Merging #803 (d887c1b) into master (d62a828) will increase coverage by 0.00%.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##           master     #803   +/-   ##
=======================================
  Coverage   46.94%   46.94%           
=======================================
  Files         146      146           
  Lines       59356    59358    +2     
=======================================
+ Hits        27862    27863    +1     
- Misses      31494    31495    +1     

Impacted Files	Coverage Δ
crates/engine_xetex/xetex/xetex-output.c	82.56% <66.66%> (-0.17%)	⬇️

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[pkgw]

Thanks!