Fix failing test (#3065)

Summary:
# Before submitting

- [ ] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
- [ ] Did you read the [contributor guideline](https://github.com/pytorch/fairseq/blob/main/CONTRIBUTING.md)?
- [ ] Did you make sure to update the docs?
- [ ] Did you write any new necessary tests?

## What does this PR do?
Fixes # (issue).

## PR review
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Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/3065

Reviewed By: Mortimerp9

Differential Revision: D34144674

Pulled By: dianaml0

fbshipit-source-id: 842b0d29c9c85d4b56b640f2823fcb4e3f912f98

tests/test_lm_context_window.py

@@ -34,6 +34,7 @@ class TestLMContextWindow(unittest.TestCase):
             dataset=dataset,
             batch_size=1,
             context_window=2,
+            num_workers=0,
         )
 
         batch = next(eval_dataloader)