Loose the packet count check in DnsServerSelection

To prioritize the DNS servers, the test sends 50 sequential DNS queries.
The number seems not enough when the RTT of the DNS server is too higher
than the test expects.

Bug: 183467297
Test: ResolverTest.DnsServerSelection passed
Change-Id: Id2fa97fd1bede4fcbe852772c76a7cefd6966a89