Decrease test time / build time

[Chriscbr]

There are a lot of tests for jsii-docgen, which will take longer to run as we add more languages. Currently they are taking more than 4 or 5 minutes on my development machine. I wonder if we can find some ways to optimize this.

One idea that comes to mind is to skip invoking jsii-rosetta during tests, since it does not really validate the core function of jsii-docgen and might be taking up a significant portion of the test time.

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[Chriscbr]

Partially addressed by #559 and #553