patch 8.2.1043: %a item in 'statusline' not tested

Problem:    %a item in 'statusline' not tested.
Solution:   Add a test. (Dominique Pellé, closes #6318)

src/testdir/test_statusline.vim

@@ -64,7 +64,17 @@ endfunc
 func Test_statusline()
   CheckFeature quickfix
 
-  new Xstatusline
+  " %a: Argument list ({current} of {max})
+  set statusline=%a
+  call assert_match('^\s*$', s:get_statusline())
+  arglocal a1 a2
+  rewind
+  call assert_match('^ (1 of 2)\s*$', s:get_statusline())
+  next
+  call assert_match('^ (2 of 2)\s*$', s:get_statusline())
+  e Xstatusline
+  call assert_match('^ ((2) of 2)\s*$', s:get_statusline())
+
   only
   set laststatus=2
   set splitbelow

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1043,
 /**/
     1042,
 /**/