Improved test coverage of if_tcl.c

[dpelle]

This PR improves coverage of Tcl tests.

[codecov-io]

Codecov Report

Merging #3140 into master will increase coverage by 0.35%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3140      +/-   ##
==========================================
+ Coverage   76.03%   76.38%   +0.35%     
==========================================
  Files          93       93              
  Lines      136706   136733      +27     
==========================================
+ Hits       103942   104448     +506     
+ Misses      32764    32285     -479

Impacted Files	Coverage Δ
src/os_unix.c	57.42% <0%> (-0.14%)	⬇️
src/gui_gtk_x11.c	48.13% <0%> (-0.1%)	⬇️
src/undo.c	83.11% <0%> (-0.07%)	⬇️
src/term.c	59.78% <0%> (-0.06%)	⬇️
src/gui.c	49.63% <0%> (-0.06%)	⬇️
src/version.c	82.95% <0%> (ø)	⬆️
src/quickfix.c	93.25% <0%> (ø)	⬆️
src/regexp.c	84.46% <0%> (+0.03%)	⬆️
src/option.c	85.03% <0%> (+0.04%)	⬆️
src/userfunc.c	87.18% <0%> (+0.05%)	⬆️
... and 7 more

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[k-takata]

I'd like to inspect the code
coverage in Codecov before it can be merged in master.

Just FYI, you can enable Travis CI and Codecov on your forked repository.
If you enable them, you can check the coverage without creating a PR here.