e2e: add rollback, pause/resume, and non-existing image tests

[ptalgulk01]

Fixes #69 (items 1, 2, 3) : Add three new e2e test scenarios from #69:

• Rollback (item 1): Extend TestUpdateReboot with phases 7-8 that roll back to the original image and verify the node returns to Idle with the original digest
• Pause/Resume (item 2): Add TestPauseResume that starts an update with paused=true, verifies the node stages but does not reboot, then resumes and verifies the update completes
• Non-existing image (item 3): Add TestNonExistingImage that updates to a corrupted digest and verifies the node enters Degraded state with Reason=Error while remaining booted on the original image

All three tests pass against a bink cluster.

Test plan

• TestControllerMembership — PASS (pre-existing)
• TestUpdateReboot (with rollback) — PASS
• TestTagResolution — PASS (pre-existing)
• TestPauseResume — PASS
• TestNonExistingImage — PASS

[ptalgulk01]

cc @alicefr @jlebon can ptal?