bridge tests: verify that we can't change MTU of bridge member interfaces

Reviewed by:	donner
Sponsored by:	Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D31305

tests/sys/net/if_bridge_test.sh

@@ -577,6 +577,11 @@ mtu_body()
 	check_mtu ${bridge} 2000
 	check_mtu ${gif} 2000
 	check_mtu ${epair}a 2000
+
+	# We're not allowed to change the MTU of a member interface
+	atf_check -s exit:1 -e ignore \
+		ifconfig ${epair}a mtu 1900
+	check_mtu ${epair}a 2000
 }
 
 mtu_cleanup()