[ci] [dask] fix test on "only one worker has data" training

[jameslamb]

Description

In #5510, we observed that the Dask tests were getting stuck and timing out after 60 minutes.

Narrowed that down to a single unit test, which is currently skipped.

LightGBM/tests/python_package_test/test_dask.py

Lines 1506 to 1507 in 0c0eb2a

	def test_training_succeeds_even_if_some_workers_do_not_have_any_data(task, output, cluster):
	pytest.skip("skipping due to timeout issues discussed in https://github.com/microsoft/LightGBM/pull/5510")

The testing work from #5510 should be continued.... that test should be re-enabled and its underlying cause should be identified and fixed.

Reproducible example

Uncomment the test linked above and push a commit to a pull request in LightGBM.

Environment info

Latest version of LightGBM (0c0eb2a).

Additional Comments

Especially see this comment describing a possible root cause of timeout issues: #5510 (comment).

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