selftests: net: use BASH for bareudp testing

In bareudp.sh, this script uses /bin/sh and it will load another lib.sh
BASH script at the very beginning.

But on some operating systems like Ubuntu, /bin/sh is actually pointed to
DASH, thus it will try to run BASH commands with DASH and consequently
leads to syntax issues:
# ./bareudp.sh: 4: ./lib.sh: Bad substitution
# ./bareudp.sh: 5: ./lib.sh: source: not found
# ./bareudp.sh: 24: ./lib.sh: Syntax error: "(" unexpected

Fix this by explicitly using BASH for bareudp.sh. This fixes test
execution failures on systems where /bin/sh is not BASH.

Reported-by: Edoardo Canepa <[e-mail]>
Link: https://bugs.launchpad.net/bugs/2129812
Signed-off-by: Po-Hsu Lin <[e-mail]>
Reviewed-by: Przemek Kitszel <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/bareudp.sh

···
-#!/bin/sh
+#!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 
 # Test various bareudp tunnel configurations.