bors: require slasher and syncing sim tests (#3645)
bors: require slasher and syncing sim tests (#3645)

## Issue Addressed
I noticed that [this build](https://github.com/sigp/lighthouse/actions/runs/3269950873/jobs/5378036501) wasn't marked failed by Bors when the `syncing-simulator-ubuntu` job failed. This is because that job is absent from the `bors.toml` config.

## Proposed Changes

Add missing jobs to Bors config so that they are required:

- `syncing-simulator-ubuntu`
- `slasher-tests`
- `disallowed-from-async-lint`

The `disallowed-from-async-lint` was previously allowed to fail because it was considered beta, but I think it's stable enough now we may as well require it.




sigp/lighthouse,sigp/lighthouse,sigp/lighthouse,sigp/lighthouse