Fix l10n_migrate MigrateL10nServerGroupTest tests [#3554515]

Problem/Motivation

The testRowsComplete and testDatasetsComplete tests in the MigrateL10nServerGroupTest class where set to incomplete in a previous issue to be able to merge it and have a functional pipeline in place for the module. Those tests need to be fixed.

Issue fork l10n_server-3554515

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