patch 8.2.0448: various functions not properly tested

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

src/testdir/test_spell.vim

@@ -160,6 +160,9 @@ func Test_spellsuggest()
   call assert_equal(['Third'], spellsuggest('THird', 1))
   call assert_equal(['All'],      spellsuggest('ALl', 1))
 
+  call assert_fails("call spellsuggest('maxch', [])", 'E745:')
+  call assert_fails("call spellsuggest('maxch', 2, [])", 'E745:')
+
   set spell&
 endfunc
 
@@ -1164,3 +1167,5 @@ let g:test_data_aff_sal = [
       \"SAL ZZ-                  _",
       \"SAL Z                    S",
       \ ]
+
+" vim: shiftwidth=2 sts=2 expandtab