[branch-2.11] Fix failed GroupMetadataManagerTest (#1934)

### Motivation

When running `GroupMetadataManagerTest`,
`testOffsetWriteAfterGroupRemoved` will be called after
`testOffsetTopicNumPartitionsModify`, which changes the
`offsetTopicNumPartitions` to 100 so that `consumerGroupPartitionId`
will be 74 while `groupPartitionId` is 0, and then the group metadata
will be sent to partition 0 while `scheduleLoadGroupAndOffsets` will
read from partition 74. Since no group metadata is loaded,

### Modifications

Migrate the fix in #1759 to
reset the `offsetTopicNumPartitions` before and after each method.

tests/src/test/java/io/streamnative/pulsar/handlers/kop/coordinator/group/GroupMetadataManagerTest.java

@@ -130,6 +130,7 @@ public void cleanup() throws Exception {
 
     @BeforeMethod
     protected void setUp() throws PulsarClientException, PulsarAdminException {
+        conf.setOffsetsTopicNumPartitions(numOffsetsPartitions);
         producerBuilder = pulsarClient.newProducer(Schema.BYTEBUFFER);
         readerBuilder = pulsarClient.newReader(Schema.BYTEBUFFER)
                 .startMessageId(MessageId.earliest);
@@ -167,6 +168,7 @@ protected void setUp() throws PulsarClientException, PulsarAdminException {
 
     @AfterMethod
     protected void tearDown() throws PulsarClientException {
+        conf.setOffsetsTopicNumPartitions(numOffsetsPartitions);
         if (consumer != null) {
             consumer.close();
         }