Extend workspaces spec to exercise VS Code functionality and filesystem/OS of workspace

A follow on from !164962 (merged), the Workspaces QA/E2E spec should check that VSCode loads, and a file can be created/saved from the workspace, as well as asserting other functionality/features related to the filesystem/OS of the running workspace.

Currently there is no automated testing at all of workspace VS Code and filesystem/OS performance. All of this must be manually tested.

Questions:

• Can existing VSCode page objects be re-used for this?
• Could the existing set of web-ide tests be run against a workspace VSCode?
• Can we open a VS Code terminal to exercise filesystem/OS-level concerns? E.g. that restarting a workspace preserves state in the home directory? E.g. Workspace changes (user level settings) are los... (&15769)
• Can we inspect the javascript console of the running browser to detect possible bugs in VS Code running in the context of a workspace? E.g. Forward `Access-Control-Allow-*` headers coming... (gitlab-org/workspaces/gitlab-workspaces-proxy#22)