patch 8.2.2092: Vim9: unpredictable errors for script tests

Problem:    Vim9: unpredictable errors for script tests.
Solution:   Use a different script file name for each run.
diff --git a/src/testdir/test_vim9_assign.vim b/src/testdir/test_vim9_assign.vim
index 635b4be..2f4c0d1 100644
--- a/src/testdir/test_vim9_assign.vim
+++ b/src/testdir/test_vim9_assign.vim
@@ -953,6 +953,7 @@
       call Func()
   [END]
   CheckScriptFailure(lines, 'E990:')
+  delfunc! g:Func
 enddef
 
 def Test_let_func_call()