[Python] Fix non-deterministic row order failure in dataset tests

[asfimport]

#6961 (comment)

Reporter: Krisztian Szucs / @kszucs
Assignee: Joris Van den Bossche / @jorisvandenbossche

_{Note: This issue was originally created as ARROW-8526. Please see the migration documentation for further details.}

[asfimport]

Krisztian Szucs / @kszucs:
Issue resolved by pull request 6989
#6989