selftests/net: convert fib-onlink-tests.sh to run it in unique namespace

Remove PEER_CMD, which is not used in this test

Here is the test result after conversion.

 ]# ./fib-onlink-tests.sh
 Error: ipv4: FIB table does not exist.
 Flush terminated
 Error: ipv6: FIB table does not exist.
 Flush terminated

 ########################################
 Configuring interfaces

   ...

     TEST: Gateway resolves to wrong nexthop device - VRF      [ OK ]

 Tests passed:  38
 Tests failed:   0
Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20231213060856.4030084-11-liuhangbin@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/fib-onlink-tests.sh

@@ -3,6 +3,7 @@
 
 # IPv4 and IPv6 onlink tests
 
+source lib.sh
 PAUSE_ON_FAIL=${PAUSE_ON_FAIL:=no}
 VERBOSE=0
 
@@ -74,9 +75,6 @@ TEST_NET4IN6[2]=10.2.1.254
 # mcast address
 MCAST6=ff02::1
 
-
-PEER_NS=bart
-PEER_CMD="ip netns exec ${PEER_NS}"
 VRF=lisa
 VRF_TABLE=1101
 PBR_TABLE=101
@@ -176,8 +174,7 @@ setup()
 	set -e
 
 	# create namespace
-	ip netns add ${PEER_NS}
-	ip -netns ${PEER_NS} li set lo up
+	setup_ns PEER_NS
 
 	# add vrf table
 	ip li add ${VRF} type vrf table ${VRF_TABLE}
@@ -219,7 +216,7 @@ setup()
 cleanup()
 {
 	# make sure we start from a clean slate
-	ip netns del ${PEER_NS} 2>/dev/null
+	cleanup_ns ${PEER_NS} 2>/dev/null
 	for n in 1 3 5 7; do
 		ip link del ${NETIFS[p${n}]} 2>/dev/null
 	done