vim-patch:8.2.0448: various functions not properly tested (#20926)

Problem:    Various functions not properly tested.
Solution:   Add more tests, especially for failures. (Yegappan Lakshmanan,
            closes vim/vim#5843)

0e05de4622

Cherry-pick test changes from patch 8.2.0427 and skip Test_has().
Cherry-pick Test_complete_wildmenu() change from patch 8.2.4339.

runtime/doc/builtin.txt

@@ -7261,6 +7261,7 @@ setqflist({list} [, {action} [, {what}]])		*setqflist()*
 							*setreg()*
 setreg({regname}, {value} [, {options}])
 		Set the register {regname} to {value}.
+		If {regname} is "" or "@", the unnamed register '"' is used.
 		The {regname} argument is a string.
 
 		{value} may be any value returned by |getreg()| or