networkd-test: fix resolved_domain_restricted_dns

megasearch.net was meant to be a non-existing bogus domain, and had been
for a long time. But it seems some domain grabber recently registered
it, and it's an actual thing now:

  $ host megasearch.net
  megasearch.net has address 207.148.248.143

This causes the test to fail randomly.

Use search.example.com instead which yields

  $ host search.example.com
  Host search.example.com not found: 3(NXDOMAIN)

Fixes: #18357