selftests: pmtu: disable DAD in all namespaces

Otherwise, the configured IPv6 address could be still "tentative"
at test time, possibly causing tests failures.
We can also drop some sleep along the code and decrease the
timeout for most commands so that the test runtime decreases.

v1 -> v2:
 - fix comment (Stefano)

Fixes: d1f1b9cb

 ("selftests: net: Introduce first PMTU test")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>