refactor: speed up hcl2cdk tests

[DanielMSchmidt]

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We now parallelize as much as possible and split the tests up into several files.

While CI indicates that this has limited performance gains, I'd still like to get it in since splitting up the test cases and supporting .only syntax makes it easier to iterate on hcl2cdk. Plus none of the changes is a bad idea from a code organisation perspective.