Refactor CatalogueCategoryRepo unit tests #305

[joelvdavies]

Description

Refactors catalogue category repo unit tests in same format as in #302 as part of #90. There are also some minor changes to the previous tests refactored in that PR as I noticed a few minor inconsistencies while creating this one.

• Also changed the base RepoDSL to contain utilities for _has_child_elements and _is_duplicate as they are repeated, particularly inside the CatalogueCategory unit tests as there they are used via delete and the specific has_child_elements tests.
• Added a check on the created and modified times inside systems update e2e tests. I meant to do this before but forgot.
• Changed System to system in all comments and error messages (unless its the first word) for consistency.
• Replaced self.test_helpers. with RepositoryTestHelpers. for SystemsRepo and CatalogueCategoryRepo unit tests.

Testing instructions

• [ ] Review code
• [ ] Check Actions build
• [ ] Review changes to test coverage

Agile board tracking

Closes #305

[joelvdavies]

Just gone through the comments, and have mentioned where they were already present in https://github.com/ral-facilities/inventory-management-system-api/pull/315, otherwise I fixed them in the latest commit there https://github.com/ral-facilities/inventory-management-system-api/pull/315/commits/9ea1038ca2b9fc403c4e12ad01a1cc2a52a8b80c.

[joelvdavies]

Addressed further unresolved comments on https://github.com/ral-facilities/inventory-management-system-api/pull/315/commits/1a20bbf8d8c2857249145490835f4e43a17b467e.