Extract XTTest features into a "test environment" class

[Uncommon]

Having an abstract XCTestCase subclass results in some weirdness, like XTTest showing up in the list of tests even though it has not test cases of its own.

The idea is to extract the repository setup, with its default content and file names, into a separate test environment class that can be easily instantiated and used by tests. Then all test classes can inherit from XCTestCaste directly.

[Uncommon]

Partially done with RepoActions in beae0773affaed92fd5952920542eee41aad79d2