[SPARK-14298] [ML] [MLlib] Add unit test for EM LDA disable checkpointing

[yanboliang]

What changes were proposed in this pull request?

This is follow up for #12089, add unit test for EM LDA which test disable checkpointing when set checkpointInterval = -1.

How was this patch tested?

unit test.

cc @jkbradley

[SparkQA]

Test build #55474 has finished for PR 12286 at commit acf7455.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #55475 has finished for PR 12286 at commit fa36cde.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[holdenk]

We could maybe make this a more complete test and actually check that their are no checkpoint files generated? Although that might be over kill.

[yanboliang]

@holdenk Thanks for your comments! Actually model.getCheckpointFiles.isEmpty checked that their are no checkpoint files generated.

[holdenk]

@yanboliang that makes sense. I was thinking maybe checking the actual checkpoint directory to catch anything else (or even just clearing the checkpoint directory so checkpointing fails), but if getCheckpointFiles was correct even before the fix than that is probably a fine way to test it to.

[jkbradley]

@holdenk I agree with you, but I'm worried it could fail if checkpoint deletion/cleanup does not happen quickly enough across tests in this suite. I'd prefer to leave it as is.

[jkbradley]

@yanboliang Thanks! LGTM
Merging with master