Validate test_input and test_output in block tests

[Pwuts]

Original comment: #8358 (comment)

backend/test/block/test_block.py -> test_available_blocks relies entirely on block.test_output to verify that a block works. Leaving test_output empty will let the tests always pass. To address this, test_available_blocks should validate test_input and test_output using block.input_schema and block.output_schema.