fix(vscode): update link in extensionsViewlet

Fix healthz socket

Forgot to resume. Went ahead and did the same for the test plugin
although it only sends messages and doesn't receive any.

release: v3.9.0

Unfortunately since plugins can now override the root this is necessary
or just can't log in.

Unfortunately we have existing plugins with routes there which we cannot
move.

docs: clarify Alpine install path in install.md

feat(testing): add unit tests for constants

docs(README.md): update to include requirements

feat(testing): add unit tests for src/common/http