Add one more Revalidation unit test

The test checks that the revalidation can retry probing until
it succeeds. To do that, Private DNS validation backoff time
is changable in PrivateDnsConfigurationTest.

Bug: 79727473
Bug: 188153519
Test: Test: cd packages/modules/DnsResolver && atest
Change-Id: I11fc8d9ee963dc54d051667971b51bfecd852a93