hypothesis_strategies: Ensure to generate valid directory entry name
Since rDMOD8d96dfedee34203a4118e48a6208ee507511590b, directory entry names are validated in DirectoryEntry model and thus must not contain any slash characters.
So update directory_entries_d hypothesis strategy to ensure such names are generated to avoid flaky tests.
Spotted yersteday while working on swh-model
(see Jenkins builds hstory).
Migrated from D6760 (view on Phabricator)