PR 7130 causes transforms/util_test to fail

[kennknowles]

http://github.com/apache/beam/pull/7130 seems to cause transforms/util_test to fail.

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ERROR: test_reshuffle_sliding_window (main.ReshuffleTest)
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BeamAssertException: Failed assert: [(1, [1, 2, 4]), (1, [1, 2, 4]), (2, [1, 2]), (2, [1, 2]), (3, [1]), (3, [1])] == [(1, [1]), (1, [1]), (1, [2]), (1, [2]), (1, [4]), (1, [4]), (2, [1]), (2, [1]), (2, [2]), (2, [2]), (3, [1]), (3, [1])] [while running 'before_reshuffle/Match']

Imported from Jira BEAM-6153. Original Jira may contain additional context.
Reported by: ccy.

[Abacn]

already fixed