selftests/net: l2_tos_ttl_inherit.sh: Run tests in their own netns.

This selftest currently runs half in the current namespace and half in
a netns of its own. Therefore, the test can fail if the current
namespace is already configured with incompatible parameters (for
example if it already has a veth0 interface).

Adapt the script to put both ends of the veth pair in their own netns.
Now veth0 is created in NS0 instead of the current namespace, while
veth1 is set up in NS1 (instead of the 'testing' netns).

The user visible netns names are randomised to minimise the risk of
conflicts with already existing namespaces. The cleanup() function
doesn't need to remove the virtual interface anymore: deleting NS0 and
NS1 automatically removes the virtual interfaces they contained.

We can remove $ns, which was only used to run ip commands in the
'testing' netns (let's use the builtin "-netns" option instead).
However, we still need a similar functionality as ping and tcpdump
now need to run in NS0. So we now have $RUN_NS0 for that.

Fixes: b690842d ("selftests/net: test l2 tunnel TOS/TTL inheriting")
Reported-by: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
Tested-by: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>