net: fix failure of TestCVE202133195

TestCVE202133195 fails in testing LookupSRV if /etc/resolv.conf sets the option
'ndots' larger than the number of dots in the domain name under query.

Fix the issue by making the input domain name in test codes 'rooted' to skip search
list qualifying.

Fixes #46955

Change-Id: I1909fa7e54e9c9af57623e57cafc905729ff99fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/330842


Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>