Add support for the Cadence Testing Framework

[jribbink]

Closes #486

Description

Adds support for running Cadence tests in VSCode.

• The tests are managed in a trie using a a wrapper around the vscode TestCollection. All workspace files are watched triggering granular updates to the trie to minimize usage of system resources & latency.
• Tests are run using the Flow CLI

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For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
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