Test: Replace wait_until() with wait_for()

Refactor waitForPrivateDnsValidation() to use wait_for().
The predicate is used to check whether the waiting should be
continued or not. Since waitForNat64Prefix() and popDnsEvent()
will be changed to use condition variable, writing this way
would make it easier to change them and avoid unnecessary awakenings.

Bug: 310108475
Test: resolv_integration_test passed
Change-Id: I2282450a6f3626cbb749030154b581554df7b174