ndp tests: Fix flakiness in ndp_slaac_default_route

The test sends RAs in order to test SLAAC handling in another host.
The router needs to also set net.inet6.ip6.forwarding=1, otherwise NAs
sent from it have the ROUTER flag clear, and upon receiving such an NA
the host will automatically delete routes learned from the router.

Fixes:		feda329622 ("netinet6 tests: Add a regression test for default router handling")
MFC after:	1 week
Sponsored by:	Klara, Inc.

tests/sys/netinet6/ndp.sh

@@ -152,7 +152,10 @@ ndp_slaac_default_route_body() {
 	ndp_if_up ${epair0}b
 	atf_check jexec ${jname} ifconfig ${epair0}a inet6 accept_rtadv
 
-        # Send an RA advertising a prefix.
+	# Make sure that NAs from us are flagged as coming from a router.
+	atf_check -o ignore sysctl net.inet6.ip6.forwarding=1
+
+	# Send an RA advertising a prefix.
 	atf_check -e ignore python3 $(atf_get_srcdir)/ra.py \
 	    --sendif ${epair0}b \
 	    --dst $(ndp_if_lladdr ${epair0}a ${jname}) \
@@ -167,7 +170,7 @@ ndp_slaac_default_route_body() {
 	    jexec ${jname} netstat -rn -6
 
 	# Get rid of the default route.
-	jexec ${jname} route -6 flush
+	atf_check -o ignore jexec ${jname} route -6 flush
 	atf_check -o not-match:"^default[[:space:]]+fe80:" \
 	    jexec ${jname} netstat -rn -6