patch 8.2.1803: a few failures are not tested

Problem:    A few failures are not tested.
Solution:   Test a few failures. (Dominique Pellé, closes #7075)

src/testdir/test_arglist.vim

@@ -426,6 +426,7 @@ func Test_argdelete()
   call assert_fails('argdelete', 'E610:')
   call assert_fails('1,100argdelete', 'E16:')
   call assert_fails('argdel /\)/', 'E55:')
+  call assert_fails('1argdel 1', 'E474:')
 
   call Reset_arglist()
   args a b c d
@@ -478,13 +479,16 @@ func Test_arglist_autocmd()
   new
   " redefine arglist; go to Xxx1
   next! Xxx1 Xxx2 Xxx3
-  " open window for all args
+  " open window for all args; Reading Xxx2 will change the arglist and the
+  " third window will get Xxx1:
+  "   win 1: Xxx1
+  "   win 2: Xxx2
+  "   win 3: Xxx1
   all
   call assert_equal('test file Xxx1', getline(1))
   wincmd w
   wincmd w
   call assert_equal('test file Xxx1', getline(1))
-  " should now be in Xxx2
   rewind
   call assert_equal('test file Xxx2', getline(1))
 

src/testdir/test_cmdline.vim

@@ -756,6 +756,10 @@ func Test_cmdline_complete_various()
   call feedkeys(":doautocmd User MyCmd a.c\<C-A>\<C-B>\"\<CR>", 'xt')
   call assert_equal("\"doautocmd User MyCmd a.c\<C-A>", @:)
 
+  " completion of autocmd group after comma
+  call feedkeys(":doautocmd BufNew,BufEn\<C-A>\<C-B>\"\<CR>", 'xt')
+  call assert_equal("\"doautocmd BufNew,BufEnter", @:)
+
   " completion for the :augroup command
   augroup XTest
   augroup END

src/testdir/test_json.vim

@@ -196,6 +196,7 @@ func Test_json_decode()
   call assert_fails('call json_decode("{[]:42}")', "E491:")
 
   call assert_fails('call json_decode("-")', "E491:")
+  call assert_fails('call json_decode("-1x")', "E491:")
   call assert_fails('call json_decode("infinit")', "E491:")
 
   call assert_fails('call json_decode("\"\\u111Z\"")', 'E491:')

src/testdir/test_listdict.vim

@@ -726,6 +726,8 @@ func Test_reduce()
   call assert_fails("call reduce({}, { acc, val -> acc + val }, 1)", 'E897:')
   call assert_fails("call reduce(0, { acc, val -> acc + val }, 1)", 'E897:')
   call assert_fails("call reduce('', { acc, val -> acc + val }, 1)", 'E897:')
+  call assert_fails("call reduce([1, 2], 'Xdoes_not_exist')", 'E117:')
+  call assert_fails("echo reduce(0z01, { acc, val -> 2 * acc + val }, '')", 'E39:')
 
   let g:lut = [1, 2, 3, 4]
   func EvilRemove()

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1803,
 /**/
     1802,
 /**/