patch 9.0.0363: common names in test files causes tests to be flaky

Problem:    Common names in test files causes tests to be flaky.
Solution:   Use more specific names.
diff --git a/src/testdir/test_vim9_assign.vim b/src/testdir/test_vim9_assign.vim
index b1ff392..58dde3f 100644
--- a/src/testdir/test_vim9_assign.vim
+++ b/src/testdir/test_vim9_assign.vim
@@ -2211,7 +2211,7 @@
   END
   v9.CheckScriptFailure(lines, 'E476:', 1)
 
-  edit! Xfile
+  edit! Xslfile
   lines =<< trim END
       var edit: bool
       legacy edit