Test generated code for field colour blocks

[rachel-fenichel]

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Request

Follow-up work for #2162

Add automated tests to run all of the generators for blocks in field-colour, against a variety of inputs, and compare the outcome to golden files.

Before merging, run the code in the golden files, to make sure that it compiles and runs in the target language and behaves appropriately.

Some of these tests can be pulled over from the core generator tests. The tests should run in node with a headless Blockly instance.

[rachel-fenichel]

Fixed by #2220