Params type for beforeAllSubcases is improperly specialized

[kainino0x]

Issue found by @shrekshao. This is a TypeScript type safety thing, not a serious problem, but would be nice to fix because it makes test development harder.

The t.params value in beforeAllSubcases(t => { ... }) has type TestParams & DeepReadonlyObject<...>. The TestParams & shouldn't be there, because it allows t.params.someFieldThatDoesntExist (which has type JSONWithUndefined) which should be a type error (Property 'someFieldThatDoesntExist' does not exist).

It works correctly in the test function .fn(t => { ... }) so there is some difference in how they're set up.