rm_stm_tests: fix broken test

The test is broken since commit 34ae66c that inadverdently switched
the batch generator to generate more than one batch per transaction and
that broke certain test invariants.

src/v/cluster/tests/rm_stm_tests.cc

@@ -47,18 +47,22 @@ static batches_with_identity make_rreader(
   int first_seq,
   int count,
   bool is_transactional) {
-    return batches_with_identity{
-      .id = model::
-        batch_identity{.pid = pid, .first_seq = first_seq, .last_seq = first_seq + count - 1, .record_count = count, .is_transactional = is_transactional},
-      .batches = random_batches(model::test::record_batch_spec{
-                                  .offset = model::offset(0),
-                                  .allow_compression = true,
-                                  .count = count,
-                                  .producer_id = pid.id,
-                                  .producer_epoch = pid.epoch,
-                                  .base_sequence = first_seq,
-                                  .is_transactional = is_transactional})
-                   .get()};
+    batches_with_identity result;
+    result.id = {
+      .pid = pid,
+      .first_seq = first_seq,
+      .last_seq = first_seq + count - 1,
+      .record_count = count,
+      .is_transactional = is_transactional};
+    result.batches.push_back(model::test::make_random_batch(
+      {.offset = model::offset(0),
+       .allow_compression = true,
+       .count = count,
+       .producer_id = pid.id,
+       .producer_epoch = pid.epoch,
+       .base_sequence = first_seq,
+       .is_transactional = is_transactional}));
+    return result;
 }
 
 void check_snapshot_sizes(cluster::rm_stm& stm, raft::consensus* c) {