1458: Run VS Code tests on CI r=matklad a=etaoins
This is actually much faster than I expected; it takes about 13 seconds to download VS Code and run the unit tests. This means the VS Code tests are still significantly faster than the Rust ones.
If this ends up being unreliable we can always remove it later or move it to a separate optional job.
We also need to ignore the `.vscode-test` directory when running `prettier` or it will get upset about some temporary JSON files VS Code creates.
cc @killercup
Co-authored-by: Ryan Cumming <etaoins@gmail.com>
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