Tidied the common test suite tests

[iainbeeston]

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)

[pd]

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[RST-J]

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