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

This change is needed for I4e27d6e89d3f851b5ea4f00da01e7093afa537b2.

Change-Id: Ia601ecd2e69aeb6669e0e2e5dd60135a751611e7
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 7a0b4b1..2d0cf93 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -315,7 +315,7 @@
              loggingrb/logging_test.err	strrb/strrb_test.ok		\
              codec/codec_test.ok \
              codec/codec_ecu_fr_test.ok \
-	     vty/vty_test.ok \
+	     vty/vty_test.ok vty/vty_test.err \
 	     vty/fail_not_de-indented.cfg \
 	     vty/fail_tabs_and_spaces.cfg \
 	     vty/fail_too_much_indent.cfg \