test_delta_sync_with_encryption - test criteria and scenario need to be fixed

test_delta_sync_with_encryption uses doc_writes_bytes_blip expvar value to validate whether delta sync is in action during the replication. The doc size has been written cannot represent delta sync ON/OFF in this case.

Alternatively, delta_push_doc_count can be used, and there should be multiple docs, some with encryptable properties some without. Validate the delta_push_doc_count matches the doc count whose encryptable properties are set.