patch 8.2.4601: Vim9: not enough test coverage for executing :def function

Problem:    Vim9: not enough test coverage for executing :def function.
Solution:   Add a few more tests.
diff --git a/src/testdir/test_vim9_cmd.vim b/src/testdir/test_vim9_cmd.vim
index ca63709..b15028a 100644
--- a/src/testdir/test_vim9_cmd.vim
+++ b/src/testdir/test_vim9_cmd.vim
@@ -1660,6 +1660,8 @@
   v9.CheckDefFailure(['s/from/\="x")/'], 'E488:')
   v9.CheckDefFailure(['s/from/\="x"/9'], 'E488:')
 
+  v9.CheckDefExecFailure(['s/this/\="that"/'], 'E486:')
+
   # When calling a function the right instruction list needs to be restored.
   g:cond = true
   var lines =<< trim END