Tidied the common test suite tests

I've put together 3 small tweaks to the common test suite code. In the order that the commits appear:

1. I've removed an unused variable
2. I've switched to loading the common test suite json files using the json gem, rather than `JSON::Validator.parse`, to insulate us from any bugs in our own parsing code
3. One of the loops was re-redefining the variable `test` inside a loop (it was previously defined outside the loop - so we were shadowing the original value)