Chore: Refactor tests to assert snapshots not JSON

[JamesHenry]

Naturally, this is a huge diff, but brings with it all of the benefits previously discussed.

• I have so far only created the snapshots and not yet deleted the old result.js files, purely to make this easier to review and reduce noise. To be clear, in this PR the result.js files are no longer being used, and I plan to remove them as soon as you are happy with the rest of the changes.
• I was able to greatly simplify the execution of the assertions now that it relies on consistent tooling (see the createSnapshotTestBlock() helper function below).
• The unreliable update-typescript-tests script is no longer necessary, as jest provides an effortless way to update the snapshots out of the box.

[soda0289]

LGTM! This should make it much easier to create tests.

[JamesHenry]

Thanks, @soda0289! Updated to include the removals. As well as all the .result.js files, we actually had a lot of unused fixtures in there, so I have removed those too. I will merge when it successfully builds.