Improved test coverage of syntax.c

[dpelle]

This PR improves test coverage of syntax.c:

• exercise more invalid argument errors for the :syntax and :highlight commands
• more coverage of syntax completion for :highlight command

[dpelle]

While improving test coverage of vim/src/syntax.c, I stumbled upon this
easter egg which results in an animation:

  $ vim -u NONE -c 'call feedkeys(":hi Ni \<C-D>")'

I did not add a test for it, as a test would take a few seconds.

See also:

  :Ni!

And:
https://www.youtube.com/watch?v=zIV4poUZAQo

[codecov-io]

Codecov Report

Merging #1624 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1624      +/-   ##
==========================================
+ Coverage   74.79%   74.82%   +0.02%     
==========================================
  Files          76       76              
  Lines      124860   124860              
==========================================
+ Hits        93391    93421      +30     
+ Misses      31469    31439      -30

Impacted Files	Coverage Δ
src/message.c	68.07% <0%> (-0.46%)	⬇️
src/channel.c	83.64% <0%> (-0.24%)	⬇️
src/window.c	81.08% <0%> (-0.17%)	⬇️
src/os_unix.c	58.29% <0%> (-0.15%)	⬇️
src/term.c	53.38% <0%> (-0.06%)	⬇️
src/gui_gtk_x11.c	47.39% <0%> (-0.06%)	⬇️
src/gui_beval.c	44.89% <0%> (+0.92%)	⬆️
src/syntax.c	76.72% <0%> (+1.3%)	⬆️

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