General sandbox repo

[https://chromium.googlesource.com/chromiumos/infra/dummies/general-sandbox]

Motivation

These repositories will not include any real code, but can be used to test or monitor aspects of the infrastructure.

For example, we may have some dummy repositories for:

• Testing git merge handling in the CQ
• New CQ submission / rejection / exoneration behaviors
• Blackbox monitoring of pre-cq or cq submission

New CQ features could be added behind a config flag, and rolled out to dummy repos first to minimize risk.

Pros:

• Easily accomplished.
• Allows for safer rollouts of new CQ features.

Cons:

• Encourages testing in production. (rebuttal: we already test a lot in production due to lack of a staging environment that replicates the real CQ environment; this just makes such inevitable tests safer).
• Adds some junk repos to the manifest.