[Testing]: fix VSCODE_PROXY_URI flakiness

[jsjoeio]

What is your suggestion?

To improve the reliability of the extensions.test.ts test should have access to VSCODE_PROXY_URI which has been failing more than usual in CI lately.

Why do you want this feature?

More confidence in e2e tests and faster working cycle.

Are there any workarounds to get this functionality today?

Re-run the tests manually when they fail.

Are you interested in submitting a PR for this?

Yes.

[jsjoeio]

One idea to fix this is to show a notification when the activate function is called meaning the extension loaded.