Manually add directly-connected routes as well as default routes

Manually add directly-connected routes as well as default routes

Currently, the manual routing configuration that we use when
running the tests on a kernel that does not have the
accept_ra_rt_table sysctl does not add directly-connected routes
to the per-interface routing tables. This does not allow us to
test on-link connectivity unless the sysctl is present.

Remove this restriction and enable testOnlinkCommunication.

Also make testLeftoverRoutes conditional on HAVE_AUTOCONF_TABLE,
because without per-interface routing tables, sending an RA does
not result in the creation of a new routing table.

Change-Id: I16177cf374040b87a78e7455b05b11b956f4e7ee