Wikibase is not an appropriate repository for a generic page split tester. This task tracks the effort to move the code from Wikibase to Core and also a few minor improvements:
- A **rename** should be considered. Perhaps PageSplitter / PageSplitterTest? This might remove some ambiguity around whether PageSplitTester is for unit tests or production code but may decrease grep visibility for A/B or split test code.
- Make the **bucket order** either more explicit or irrelevant. I had to carefully note the order used for config patches and calculate the page_random values for that given order.
- Add **documentation and examples** (some of this lives in the tests), clarify what’s the same and what differs from client A/B tests, clarify that bucket assignments are monotonic.
= Acceptance criteria
 No PageSplitTester / PageSplitTesterTest or PageRandomLookup / PageRandomLookupTest code, configuration, or strings in Wikibase
 PageSplitTester (or whatever it's now called) exists in Core and can be used by any extension
 Bucket order specified by the config is as clear and safe as possible
 Documentation is revised and added where needed