Since rDMOD8d96dfedee34203a4118e48a6208ee507511590b, directory entry name
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](https://jenkins.softwareheritage.org/job/DMOD/job/tests/)).