patch 8.2.2831: Vim9: expandcmd() not tested

Problem:    Vim9: expandcmd() not tested.
Solution:   Add a test.

src/testdir/test_vim9_builtin.vim

@@ -336,6 +336,13 @@ def Test_expand()
   close
 enddef
 
+def Test_expandcmd()
+  $FOO = "blue"
+  assert_equal("blue sky", expandcmd("`=$FOO .. ' sky'`"))
+
+  assert_equal("yes", expandcmd("`={a: 'yes'}['a']`"))
+enddef
+
 def Test_extend_arg_types()
   g:number_one = 1
   g:string_keep = 'keep'

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2831,
 /**/
     2830,
 /**/