Pez/vsix+fs testing flakyness

[PEZ]

• Created a dummy extension to feed the vscode-test launcher when we are testing the extension from the filesystem

• Installing extensions and user data in the local VS Code Insiders (not the Machine/global install as before). Should make it safer and more stable to run the tests locally.

• Fixes #154

• [x] I have read the developer documentation.

• [x] This PR corresponds to an issue with a clear problem statement.

• [x] This PR contains a test to prevent against future regressions

• [x] I have updated the [Unreleased] section of the CHANGELOG.md file with a link to the addressed issue.

[borkdude]

@PEZ I'll re-run this a few times to double-check on the flakiness :-D

[borkdude]

Other than my question: LGTM!