Fix a test bug that doesn't restart network when using experiment flag

Since the resolver updates the flags only when a network is created or
destroyed, it's necessary to restart the testing network after
ScopedSystemProperties.

Also add a test that passes only if sort_nameservers is enabled.

Bug: 137169582
Test: ResolverTest#SkipBadServersDueToInternalError passed
      ResolverTest#SkipBadServersDueToTimeout passed
      ResolverTest#DnsServerSelection passed
Change-Id: I81b37de7fd6fd09591793bf5e079f5109ab41d0c