[tests] tailscale integration test

Is your feature request related to a problem? Please describe

We lack integration tests that tell us about the health of our support for the tailscale feature.

Describe the behaviour you'd like

Our integration tests should assert that the tailscale integration, including support for DNS, is working as expected.

Describe alternatives you've considered

Continue doing manual testing.

Additional context

https://github.com/gitpod-io/workspace-images/pull/956
https://github.com/gitpod-io/website/pull/2908
https://github.com/gitpod-io/gitpod/issues/13785

Adding secrets to support this test.

1. Add the secret to GCP (manually) in gitpod-core-dev project
2. Copy the secret from GCP to core-dev with TF (docs)
3. Copy the secret from core-dev to the preview environment as part of the deploy (example)

The secret you'll be storing is a tailscale key (authkey). It should be reusable (so it can support many preview VMs) and ephemeral (so when machines go offline from the tailscale network, that they are removed).

Start by reviewing the linked workspace-images, website, and gitpod issues, then inspect dev/preview/workflow/preview/deploy-gitpod.sh and the referenced secret-deployment steps. Identify the existing integration-test entry point and determine how preview environments receive the Tailscale key. Done means integration tests cover Tailscale support, including DNS, with the required reusable and ephemeral secret configuration.