Improve E2E tests to verify workspace setup via VS Code Terminal

MR: Pending

Description

NOTE: This is a followup from this comment on Setup integration tests in the Web IDE (gitlab-web-ide#6 - closed) • Enrique Alcántara • 17.11.

The Workspaces E2E/QA specs do not actually test any behavior from within the workspace.

Building on work done for the Web IDE integration testing in Setup integration tests in the Web IDE (gitlab-web-ide#6 - closed) • Enrique Alcántara • 17.11 and other issues, we could leverage this for workspaces too.

E.g., opening the terminal in VS Code, and asserting on things specified in the devfile, like filesystem mounts, postStart scripts, file/env secret settings, etc.

Acceptance criteria

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Implementation plan

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