selftests: openvswitch: Use bash as interpreter

openvswitch.sh makes use of substitutions of the form ${ns:0:1}, to
obtain the first character of $ns. Empirically, this is works with bash
but not dash. When run with dash these evaluate to an empty string and
printing an error to stdout.

 # dash -c 'ns=client; echo "${ns:0:1}"' 2>error
 # cat error
 dash: 1: Bad substitution
 # bash -c 'ns=client; echo "${ns:0:1}"' 2>error
 c
 # cat error

This leads to tests that neither pass nor fail.
F.e.

 TEST: arp_ping                                                      [START]
 adding sandbox 'test_arp_ping'
 Adding DP/Bridge IF: sbx:test_arp_ping dp:arpping {, , }
 create namespaces
 ./openvswitch.sh: 282: eval: Bad substitution
 TEST: ct_connect_v4                                                 [START]
 adding sandbox 'test_ct_connect_v4'
 Adding DP/Bridge IF: sbx:test_ct_connect_v4 dp:ct4 {, , }
 ./openvswitch.sh: 322: eval: Bad substitution
 create namespaces

Resolve this by making openvswitch.sh a bash script.

Fixes: 918423fd ("selftests: openvswitch: add an initial flow programming case")
Signed-off-by: Simon Horman <horms@kernel.org>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Link: https://lore.kernel.org/r/20240617-ovs-selftest-bash-v1-1-7ae6ccd3617b@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/openvswitch/openvswitch.sh

-#!/bin/sh
+#!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 #
 # OVS kernel module self tests