t/tfault-run: Require bash

Otherwise when /bin/sh is dash it fails with

    t/tfault-run: 35: test: on_pagefault: unexpected operator

t/tfault-run

-#!/bin/sh -e
+#!/bin/bash -e
 # tfault-run <tfault> <arg> <mustdie>
 # run `<tfault> <arg>` and verify that it produces correct coredump, dieing for
 #   SIGSEGV in function <mustdie>.