Validate test case input and output at runtime

[echeran]

Now that we have schemas for test input and output, we should enable runtime validation of those test inputs & outputs across the board.

Doing so will enable the realization of a large chunk of the value proposition for having the schemas. It would ensure that all test cases passed to executors, and all data received from executors, adhere to the contracts defined by the schemas.

[sffc]

Action: make sure we do this consistently everywhere.