Set pipefail for wrapper scripts

Previously if the testr list command was failing subunit2py would
return a 0 making the test runs that use the pretty_tox scripts also
return 0 despite testr actually failing. This commit gets around this
condition by setting pipefail in the pretty_tox scripts to ensure that
we actually fail.

Change-Id: I9cbaf83cfea7c02133b27fe639e7ea12edb48f7e
diff --git a/tools/pretty_tox_serial.sh b/tools/pretty_tox_serial.sh
index 45f05bd..42ce760 100755
--- a/tools/pretty_tox_serial.sh
+++ b/tools/pretty_tox_serial.sh
@@ -1,4 +1,6 @@
-#!/bin/sh
+#!/usr/bin/env bash
+
+set -o pipefail
 
 TESTRARGS=$@