patch 8.1.1796: :argdo is not tested

Problem:    :argdo is not tested
Solution:   Add a test.

src/testdir/test_arglist.vim

@@ -496,3 +496,12 @@ func Test_large_arg()
   exe 'argadd ' .repeat('x', &columns)
   args
 endfunc
+
+func Test_argdo()
+  next! Xa.c Xb.c Xc.c
+  new
+  let l = []
+  argdo call add(l, expand('%'))
+  call assert_equal(['Xa.c', 'Xb.c', 'Xc.c'], l)
+  bwipe Xa.c Xb.c Xc.c
+endfunc

src/version.c

@@ -773,6 +773,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1796,
 /**/
     1795,
 /**/