test/CI performance: vscode downloaded multiple times

# Problem

Since we now run the tests in each `package/x/` separately, vscode is downloaded redundantly in our tests:

```
Running unit test suite...
Setting up VS Code test instance, version: stable
Downloading VS Code 1.91.1 from https://update.code.visualstudio.com/1.91.1/darwin-arm64/stable
Downloading VS Code (130067222B)
```

This adds time to each CI run.

# Proposal

vscode-test has a flag that allows pointing to a directory where it should be downloaded (or skipped if it already exists). Update our test runner script to specify a directory at the repo top level.

Find the test runner script used by the separate package/x/ test runs and read how vscode-test sets up its instance. Check the vscode-test option for a shared download directory at the repository top level, then verify that repeated test runs reuse the existing VS Code download and that CI still completes successfully.

Written by the indexing model from the issue text.