Make the MTU tests more realistic.

Currently, the MTU tests work on networks that have not yet been
connected. This is not very realistic because most MTU changes
(like most LinkProperties changes in general) happen after the
network has connected. Change these tests to use connected
networks instead.

To avoid losing coverage for setting the MTU before the network
is connected, add a test just for that.

Bug: 143158421
Test: test-only change
Change-Id: I7f808249722924a0b1253e402c5580bb104afd8e