tests/multiboot: Improve portability by searching bash in the $PATH

Bash is not always installed as /bin/bash. In particular on OpenBSD,
the package installs it in /usr/local/bin.
Use the 'env' shebang to search bash in the $PATH.

Reviewed-by: Kamil Rytarowski <n54@gmx.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/multiboot/run_test.sh

-#!/bin/bash
+#!/usr/bin/env bash
 
 # Copyright (c) 2013 Kevin Wolf <kwolf@redhat.com>
 #