tests: ipv6: Fix DAD timeout test

DAD timeout was wrongly checking the reply order. The code will always
assign sequentially the reply to 0-2 with the current uptime. This means
that we will always have dad[0] < dad[1] < dad[2]. Check it is useless.
Instead, let just check if we got all replies.

The test checking the time between the first and last request is to
fragile. It is testing a constant independently of the tested platform
failing in several of them. Just remove it.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>