net: use case-insensitive host string comparison in TestLookup*

Some nameservers alter the case of records as they return, e.g
.google.COM or .Google.com.
However according to RFC4343, DNS name should be treated in case insensitive fashion.
This CL will fix case sensitive testcases.

Fixes #34781

Change-Id: I5f9f6a41ddc1c61993e8d1f934ef0febddc3adc1
Reviewed-on: https://go-review.googlesource.com/c/go/+/200277


Reviewed-by: Andrei Tudor Călin <mail@acln.ro>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>