unittests,syntax: Check for sanity of highlight_init_cmdline

Also fixes some errors found.

test/functional/ui/cmdline_highlight_spec.lua

@@ -899,7 +899,6 @@ describe('Expressions coloring support', function()
     ]])
   end)
   -- FIXME: Test expr coloring when using -u NORC and -u NONE.
-  -- FIXME: Test all highlight groups, using long expression.
   -- FIXME: Test different ways of triggering expression highlighting (:<C-r>=,
   --        i<C-r>=, :<C-\>e, "=).
 end)