patch 8.2.1283: Vim9: error for misplaced -> lacks argument

Problem:    Vim9: error for misplaced -> lacks argument.
Solution:   Use the pointer before it was advanced.
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index b0a83f4..d7072f3 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -1532,7 +1532,7 @@
 
   call CheckDefFailure(["let x = ''", "let y = x.memb"], 'E715:')
 
-  call CheckDefFailure(["'yes'->", "Echo()"], 'E488:')
+  call CheckDefFailure(["'yes'->", "Echo()"], 'E488: Trailing characters: ->')
 
   call CheckDefExecFailure(["[1, 2->len()"], 'E697:')
   call CheckDefExecFailure(["#{a: 1->len()"], 'E488:')