Add orchestrator code for built-in implementation Z6926
Z6926 = Built-in implementation of Fetch Wikidata lexeme sense
The new tests for sense fetching are modeled after the existing tests for form fetching.
Moved a few things around in dbtest.js, to have a better ordering / grouping. Makes it harder to review; apologies.
Note: most of the fetching work (retrieving from Wikidata and converting into a ZObject) is done by pre-existing functions, which are already in use for getting senses nested within lexemes. This patch completes the built-in for independently fetching a sense, given its ID.
Bug: T380340
Closes T380340