check-in: return test helpers info and deprecate /api/v1/test-helpers

[bassosimone]

This issue is about improving performance by obtaining test helpers info via check-in.

Here's what we need to do more in detail:

• backend: modify /api/v1/check-in to return test helpers info
• sysadmin: deploy new backend
• probe: use /api/v1/check-in to obtain test helpers info
• probe: drop code to call /api/v1/test-helpers
• backend: deprecate /api/v1/test-helpers

We discussed this topic internally with @hellais and @FedericoCeratto a few days ago.

More specifically, we will add the following key to the check-in API response:

{
  "conf": {
    "test_helpers": {
      /* Information currently returned by /api/v1/test-helpers */
    }
  }
}