docs(cdk/testing): add documentation for harness environment authors

[mmalerba]

No description provided.

[jelbourn]

Here's my proposed edits: jelbourn@da6225e

[mmalerba]

Mostly fine with the edits, but I did add back the bit at the beginning describing what a "harness environment author" is. Earlier in these docs there's a paragraph that lays out the different classes of developer that someone might fall into (test author, harness author, harness environment author) and promises to explain more about each later

[jelbourn]

LGTM

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