patch 8.2.2805: Vim9: cannot use legacy syntax in Vim9 script

Problem:    Vim9: cannot use legacy syntax in Vim9 script.
Solution:   Add the :legacy command.
diff --git a/src/testdir/test_vim9_assign.vim b/src/testdir/test_vim9_assign.vim
index ad942a8..a0175cd 100644
--- a/src/testdir/test_vim9_assign.vim
+++ b/src/testdir/test_vim9_assign.vim
@@ -1500,33 +1500,33 @@
   # OK to define script-local later when prefixed with s:
   var lines =<< trim END
     def SetLater()
-      s:legacy = 'two'
+      s:legvar = 'two'
     enddef
     defcompile
-    let s:legacy = 'one'
+    let s:legvar = 'one'
     call SetLater()
-    call assert_equal('two', s:legacy)
+    call assert_equal('two', s:legvar)
   END
   CheckScriptSuccess(lines)
 
   # OK to leave out s: prefix when script-local already defined
   lines =<< trim END
-    let s:legacy = 'one'
+    let s:legvar = 'one'
     def SetNoPrefix()
-      legacy = 'two'
+      legvar = 'two'
     enddef
     call SetNoPrefix()
-    call assert_equal('two', s:legacy)
+    call assert_equal('two', s:legvar)
   END
   CheckScriptSuccess(lines)
 
   # Not OK to leave out s: prefix when script-local defined later
   lines =<< trim END
     def SetLaterNoPrefix()
-      legacy = 'two'
+      legvar = 'two'
     enddef
     defcompile
-    let s:legacy = 'one'
+    let s:legvar = 'one'
   END
   CheckScriptFailure(lines, 'E476:', 1)
 enddef