Make line_completion() testable

I think this is a revert of some previous change made by mistake.
Use vifm_wcwidth() in column_view/utf8.c
Add missing test function declaration
Don't define Windows only test function on *nix
Remove unused exec_dummy() local function
Add/fix includes in tests
Fix copy&paste error in viewcolumns_parser tests

One of tests wasn't called because of mistake.
Merge branch 'fix-more-tests-build-issues'

This continues work started in the previously merged branch, turned out
that it wasn't completed.  Full clean&rebuild of unit tests was
required.