Travis failures with Postgres tests

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Description

Travis builds are failing semi randomly now that we're testing against Postgres. Tests pass when rerun manually. I suspect a database contention or cleaning issue, but I haven't reproduced it reliably yet.

Beginne mit der Untersuchung der Travis-Konfiguration und der Postgres-Testeinrichtung und führe dann die fehlschlagende Testsuite erneut aus, um sporadische Fehler mit manuellen Wiederholungen zu vergleichen. Untersuche die gemeldete Datenbankkonkurrenz oder das Bereinigungsverhalten; als erledigt gilt die Aufgabe, wenn die Postgres-Tests in Travis konsistent bestehen.