Tweak the format of the test scripts created by render.py

This MR modifies the format of the test scripts generated by render.py to itemise all of the tests in a list variable and then pass that to the pytest.markp.parametrize decorator, rather than creating the list directly inside the decorator call. This should mean that when tests fail, the traceback only includes a single line that shows the decorator call, and not hundreds of lines with the list of created tests.