This PR adds the following sync tests to CI workflow - triggered when a PR is labeled `syncing` - to ensure we have some e2e coverage on basic sync scenarios:
- [x] checkpoint sync to a live network (covers range and backfill sync for _current_ fork)
- [x] checkpoint sync to a running devnet (covers range and backfill sync for _next_ fork)
It seems to work fine running on github hosted runners - but if performance become an issue we could switch to using self hosted runners for sepolia sync test. (standard CPU runners have 4 CPU, 16 GB ram - i think it _should_ be enough on sepolia / devnet networks)
The following tests have been **removed** from this PR and moved to a separate issue *(#7550)
- [x] genesis sync on a local devnet (covers current and next fork)
- [x] brief shutdown and restart (covers lookup sync)
- [x] longer shutdown and restart (covers range sync)
I'm hoping to keep these e2e test maintenance effort to a minimum - hopefully longer term we could have some generic e2e tests that works for all clients and the maintenance effort can be spread across teams.
### Latest test run:
https://github.com/sigp/lighthouse/actions/runs/15411744248
### Results:
<img width="687" alt="image" src="https://github.com/user-attachments/assets/c7178291-7b39-4f3b-a339-d3715eb16081" />
<img width="693" alt="image" src="https://github.com/user-attachments/assets/a8fc3520-296c-4baf-ae1e-1e887e660a3c" />
#### logs are available as artifacts:
<img width="629" alt="image" src="https://github.com/user-attachments/assets/3c0e1cd7-9c94-4d0c-be62-5e45179ab8f3" />
