patch 8.2.3990: testing wrong operator

Problem:    Testing wrong operator.
Solution:   Test "g@" instead of "r_". (Naohiro Ono, closes #9463)
diff --git a/src/testdir/test_normal.vim b/src/testdir/test_normal.vim
index 90b7f9d..4a09964 100644
--- a/src/testdir/test_normal.vim
+++ b/src/testdir/test_normal.vim
@@ -486,7 +486,7 @@
   set operatorfunc=Underscorize
   new
   call setline(1, ['first', 'first', 'third', 'third', 'second'])
-  normal! 1GVjr_
+  normal! 1GVjg@
   normal! 5G.
   normal! 3G.
   call assert_equal(['_____', '_____', '_____', '_____', '______'], getline(1, '$'))