tests: do not ignore stderr of vty_test, also match it

This change is needed for I4e27d6e89d3f851b5ea4f00da01e7093afa537b2.

Change-Id: Ia601ecd2e69aeb6669e0e2e5dd60135a751611e7
diff --git a/tests/testsuite.at b/tests/testsuite.at
index 493c16f..8b6a20a 100644
--- a/tests/testsuite.at
+++ b/tests/testsuite.at
@@ -204,8 +204,9 @@
 AT_SETUP([vty])
 AT_KEYWORDS([vty])
 cat $abs_srcdir/vty/vty_test.ok > expout
+cat $abs_srcdir/vty/vty_test.err > experr
 cp $abs_srcdir/vty/*.cfg .
-AT_CHECK([$abs_top_builddir/tests/vty/vty_test], [0], [expout], [ignore])
+AT_CHECK([$abs_top_builddir/tests/vty/vty_test], [0], [expout], [experr])
 AT_CLEANUP
 
 AT_SETUP([gprs-bssgp])