Refactor all test cases into 'looped' tests

[RufusJWB]

@sleevi I followed your proposal and refactored test test into a loop. To make this whole thing more re-useable and minimize the amount of duplicated code, I moved the definition of the TestCase struct and of the RunTest func into https://github.com/RufusJWB/zlint/blob/master/v2/test/helpers.go . Now they can be re-used wherever needed in the code.

Originally posted by @RufusJWB in #472 (comment)

[cpu]

I moved the definition of the TestCase struct

I think it's fairly idiomatic to define this on a per-test basis. I wouldn't worry too much about duplication here.

I'm supportive of reworking existing tests into the "table driven" format favoured by the Go community but it's a high labour low reward task. If someone feels up to I won't try to dissuade them but it's also a lot of work without a ton of functional gains.

[RufusJWB]

@cpu I'll create a branch based on this issue, so that the work I've done already will be stored and revert the changes in the #472

[cpu]

Thanks @RufusJWB, sorry for the churn.