add a netd_integration_test to ensure correct labelling of /sys/class/net/*/mtu files

Test: atest netd_integration_test (on cuttlefish)
[55/57] NetdSELinuxTest#CheckProperMTULabels: PASSED (26ms)

Bug: 131863575
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: If8da0a5f3f6c9faa632fdd64446949e635ffe42f