tx/ducktape: breakup transactions_test.py monolith

[bharathv]

No functional changes, just moved code around to break up the huge file that is difficult to maintain in the current form.

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v24.3.x
• v24.2.x
• v24.1.x

Release Notes

• none

[vbotbuildovich]

CI test results

test results on build#60867

test_id	test_kind	job_url	test_status	passed
rptest.tests.partition_reassignments_test.PartitionReassignmentsTest.test_reassignments_kafka_cli	ducktape	https://buildkite.com/redpanda/redpanda/builds/60867#01947118-8d42-4907-b299-6211dbd07de8	FLAKY	1/2
rptest.tests.partition_reassignments_test.PartitionReassignmentsTest.test_reassignments_kafka_cli	ducktape	https://buildkite.com/redpanda/redpanda/builds/60867#0194711c-ab7f-48c5-8f45-4d7825451085	FLAKY	1/2

test results on build#60917

test_id	test_kind	job_url	test_status	passed
rptest.tests.datalake.partition_movement_test.PartitionMovementTest.test_cross_core_movements.cloud_storage_type=CloudStorageType.S3	ducktape	https://buildkite.com/redpanda/redpanda/builds/60917#01947594-f60d-405f-bb8d-2db01141f9b1	FLAKY	1/2