vim-patch:9.1.0444: Not enough tests for getregion() with multibyte chars (#29000)

Problem:  Not enough tests for getregion() with multibyte chars.
Solution: Add a few more tests (zeertzjq).

closes: vim/vim#14844

dff55a3358

src/nvim/eval.lua

@@ -4372,7 +4372,8 @@ M.funcs = {
 
       	type		Specify the region's selection type.
       			See |getregtype()| for possible values,
-      			except it cannot be an empty string.
+      			except that the width can be omitted
+      			and an empty string cannot be used.
       			(default: "v")
 
       	exclusive	If |TRUE|, use exclusive selection