patch 9.0.1496: test restoring register with wrong value

Problem:    Test restoring register with wrong value.
Solution:   Correct name of variable. (closes #12310)
diff --git a/src/testdir/test_termcodes.vim b/src/testdir/test_termcodes.vim
index 8e8be02..7e450d9 100644
--- a/src/testdir/test_termcodes.vim
+++ b/src/testdir/test_termcodes.vim
@@ -266,7 +266,7 @@
   let &ttymouse = save_ttymouse
   call test_override('no_query_mouse', 0)
   let @* = save_quotestar
-  let @+ = save_quotestar
+  let @+ = save_quoteplus
   bwipe!
 endfunc