selftests: netdevsim: improve lib.sh include in peer.sh

Fix the peer.sh test to run from INSTALL_PATH.

Signed-off-by: David Wei <dw@davidwei.uk>
Link: https://patch.msgid.link/20250617212102.175711-2-dw@davidwei.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: spikeh

tools/testing/selftests/drivers/net/netdevsim/peer.sh

@@ -1,7 +1,8 @@
 #!/bin/bash
 # SPDX-License-Identifier: GPL-2.0-only
 
-source ../../../net/lib.sh
+lib_dir=$(dirname $0)/../../../net
+source $lib_dir/lib.sh
 
 NSIM_DEV_1_ID=$((256 + RANDOM % 256))
 NSIM_DEV_1_SYS=/sys/bus/netdevsim/devices/netdevsim$NSIM_DEV_1_ID