Test: Some comments and small fixes for PrivateDnsDohTest am: cdb5bbc9 am: d98324b5 am: 92c6d135

Original change: https://android-review.googlesource.com/c/platform/packages/modules/DnsResolver/+/2447582



Change-Id: I2f40f9efbfd6d0c885a14f90dcfcae10fc099952
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>