net: use case-insensitive host string comparison in TestLookupGmailNS

net: use case-insensitive host string comparison in TestLookupGmailNS

Some nameservers alter the case of NS records they return, e.g.
ns2.google.COm. or ns2.google.coM. Change TestLookupGmailNS to account
for this possibility by comparing host names in lower case.

Fixes #34446

Change-Id: I6ccb5b87b42401e04c9b32cecb8b7b4267b654cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/196801
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>