tests: flush_queue_test: de-template
tests: flush_queue_test: de-template

The internal test_propagation template is instantiated many times. Replace
with an oridinary function to reduce bloat. Call sites adjusted to have a
uniform signature.

avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla