[Question] Access to the PUBSUB unit test error

### What happened?

My scene is to access PUBSUB as data source
`
pipeline.apply("readpubsub", PubsubIO.readMessageWithMessageId().fromSubscription(sriptionid))...
`

Just as the unit test case is written, it is also injected into the Pubsubmessage object, but an error is reported
junit test code:
`
PubsubMessage pubsubMessage = Mockito.mock(PubsubMessage.class);
....
PCollection out = pipeline.apply("input", Create.of(pubsubMessage)).apply(....)
`

error: Unable to infer a coder and no Coder was specified.
I want to know how to set the coder about how to set the custom object pipeline

### Issue Failure

Failure: Test is flaky

### Issue Priority

Priority: 3

### Issue Component

Component: beam-community