selftests: net: Fix flaky neighbor garbage collection test

The purpose of the "Periodic garbage collection" test case is to make
sure that "extern_valid" neighbors are not flushed during periodic
garbage collection, unlike regular neighbor entries.

The test case is currently doing the following:

1. Changing the base reachable time to 10 seconds so that periodic
garbage collection will run every 5 seconds.

2. Changing the garbage collection stale time to 5 seconds so that
neighbors that have not been used in the last 5 seconds will be
considered for removal.

3. Waiting for the base reachable time change to take effect.

4. Adding an "extern_valid" neighbor, a non-"extern_valid" neighbor and
a bunch of other neighbors so that the threshold ("thresh1") will be
crossed and stale neighbors will be flushed during garbage
collection.

5. Waiting for 10 seconds to give garbage collection a chance to run.

6. Checking that the "extern_valid" neighbor was not flushed and that
the non-"extern_valid" neighbor was flushed.

The test sometimes fails in the netdev CI because the non-"extern_valid"
neighbor was not flushed. I am unable to reproduce this locally, but my
theory that since we do not know exactly when the periodic garbage
collection runs, it is possible for it to run at a time when the
non-"extern_valid" neighbor is still not considered stale.

Fix by moving the addition of the two neighbors before step 3 and by
reducing the garbage collection stale time to 1 second, to ensure that
both neighbors are considered stale when garbage collection runs.

Fixes: 171f2ee31a42 ("selftests: net: Add a selftest for externally validated neighbor entries")
Reported-by: Jakub Kicinski <[e-mail]>
Closes: https://lore.kernel.org/netdev/[e-mail]/
Signed-off-by: Ido Schimmel <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/test_neigh.sh

···
 	orig_base_reachable=$(ip -j ntable show name "$tbl_name" | jq '.[] | select(has("thresh1")) | .["base_reachable"]')
 	run_cmd "ip ntable change name $tbl_name thresh1 10 base_reachable 10000"
 	orig_gc_stale=$(ip -n "$ns1" -j ntable show name "$tbl_name" dev veth0 | jq '.[]["gc_stale"]')
-	run_cmd "ip -n $ns1 ntable change name $tbl_name dev veth0 gc_stale 5000"
-	# Wait orig_base_reachable/2 for the new interval to take effect.
-	run_cmd "sleep $(((orig_base_reachable / 1000) / 2 + 2))"
+	run_cmd "ip -n $ns1 ntable change name $tbl_name dev veth0 gc_stale 1000"
 	run_cmd "ip -n $ns1 neigh add $ip_addr lladdr $mac nud stale dev veth0 extern_valid"
 	run_cmd "ip -n $ns1 neigh add ${subnet}3 lladdr $mac nud stale dev veth0"
+	# Wait orig_base_reachable/2 for the new interval to take effect.
+	run_cmd "sleep $(((orig_base_reachable / 1000) / 2 + 2))"
 	for i in {1..20}; do
 		run_cmd "ip -n $ns1 neigh add ${subnet}$((i + 4)) nud none dev veth0"
 	done