MINOR: Fix failing test case in TransactionLogTest (#7895)

This patch fixes a brittle expectation on the `toString` implementation coming from `Set`. This was failing on jenkins with the following error:
```
java.lang.AssertionError: expected:<Some(producerId:1334,producerEpoch:0,state=Ongoing,partitions=Set(topic-0),txnLastUpdateTimestamp=0,txnTimeoutMs=1000)> but was:<Some(producerId:1334,producerEpoch:0,state=Ongoing,partitions=HashSet(topic-0),txnLastUpdateTimestamp=0,txnTimeoutMs=1000)>
```
Instead we convert the collection to a string directly.

Reviewers: Boyang Chen <boyang@confluent.io>, Rajini Sivaram <rajinisivaram@googlemail.com>

core/src/main/scala/kafka/coordinator/transaction/TransactionLog.scala

@@ -281,7 +281,7 @@ object TransactionLog {
       case Some(txnMetadata) => s"producerId:${txnMetadata.producerId}," +
         s"producerEpoch:${txnMetadata.producerEpoch}," +
         s"state=${txnMetadata.state}," +
-        s"partitions=${txnMetadata.topicPartitions}," +
+        s"partitions=${txnMetadata.topicPartitions.mkString("[", ",", "]")}," +
         s"txnLastUpdateTimestamp=${txnMetadata.txnLastUpdateTimestamp}," +
         s"txnTimeoutMs=${txnMetadata.txnTimeoutMs}"
     }

core/src/test/scala/unit/kafka/coordinator/transaction/TransactionLogTest.scala

@@ -123,7 +123,7 @@ class TransactionLogTest {
     val (keyStringOpt, valueStringOpt) = TransactionLog.formatRecordKeyAndValue(transactionMetadataRecord)
     assertEquals(Some(s"transaction_metadata::transactionalId=$transactionalId"), keyStringOpt)
     assertEquals(Some(s"producerId:$producerId,producerEpoch:$producerEpoch,state=Ongoing," +
-      s"partitions=Set($topicPartition),txnLastUpdateTimestamp=0,txnTimeoutMs=$transactionTimeoutMs"), valueStringOpt)
+      s"partitions=[$topicPartition],txnLastUpdateTimestamp=0,txnTimeoutMs=$transactionTimeoutMs"), valueStringOpt)
   }
 
   @Test