Fix the workspace integration tests that need a username in DB first

[jenting]

Bug description

The following integration tests need a username in DB first, however, the integration test should be able to mock the username into the DB rather than have a real username in DB.

• TestGitHubContexts
• TestWorkspaceInstrumentation
• TestGitActions

Steps to reproduce

$ go test -v ./... \
  -kubeconfig=/home/gitpod/.kube/config \
  -namespace=<namespace>

The above tests are skipped because of missing the username. The error message would be

cannot start server: "cannot access Gitpod server API: \"no suitable user found: make sure there's at least one non-builtin user in the database (e.g. login)\""

Even if we provide the username to it, the integration tests could be failed because the username doesn't in the DB.

$ go test -v ./... \
  -kubeconfig=/home/gitpod/.kube/config \
  -namespace=<namespace>
  -username=integration-test

Ref werft job

Workspace affected

No response

Expected behavior

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Example repository

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Anything else?

No response