Add parametrizable tests

[SanderMertens]

This PR adds a feature that enables parametrizable test cases. Example:

{
    "id": "MySuite",
    "params": {
        "kind": ["foo", "bar"]
    }
}

The MySuite test suite will be executed twice, once with foo as value for the kind parameter, and once with bar. Test cases can get the parameter value with test_param(param_name).