Fix establishing UDP connection

Fix establishing UDP connection

Previously, in establish_connection() for UDP protocol data were sent
but never read on peer socket. That lead to successful read on peer side
if this connection was filtered. Having constant testing string masked
this issue as we can't distinguish to which test of connectivity data
belong.

This patch makes unique data string per test_connectivity() and
also makes establish_connection() to create an ASSURED entry in
conntrack table. Finally, in last test after firewall filter was
removed, connection is re-established in order to avoid troubles with
terminated processes or TCP continuing sending packets which weren't
successfully delivered.

Closes-Bug: 1478847
Change-Id: I2920d587d8df8d96dc1c752c28f48ba495f3cf0f