patch 8.2.2831: Vim9: expandcmd() not tested

Problem:    Vim9: expandcmd() not tested.
Solution:   Add a test.
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index bd84a6c..46fe3e5 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -336,6 +336,13 @@
   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'