Integrate test mechanism to query webview DOM

A package that can be used to query the `webview` can be found here: https://github.com/redhat-developer/vscode-extension-tester. It allows for many kinds of tests and automations, including `webview` DOMs (https://github.com/redhat-developer/vscode-extension-tester/wiki/WebView). It may also be useful for testing code completion. The bundled `extest` script appears to offer the same functionality as `src/tests/runTests.ts` but also integrates selenium web drivers.