`TestMergeQuerierWithSecondaries_ErrorHandling` now tests `NewMergeQuerier`
rather than creating the data structure directly. This means we now test
short-circuiting when only a single querier is required.
Merge `mockGenericQuerier` into `mockQuerier`.
Replace `unwrapMockGenericQuerier` with a visitor pattern.
No change in functionality intended.
Signed-off-by: Bryan Boreham <bjboreham@gmail.com>