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)
diff --git a/src/testdir/test_arglist.vim b/src/testdir/test_arglist.vim
index 615d816..f45a0d4 100644
--- a/src/testdir/test_arglist.vim
+++ b/src/testdir/test_arglist.vim
@@ -426,6 +426,7 @@
   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 @@
   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))