fix(ci): make the purpose of each sync test clearer

[teor2345]

Motivation

We're about to add a full sync test in #1592, but the names of the existing tests are confusing.

Solution

Change the id, name, and add comments for each sync test.

The test from #1592 would look something like:

  # Test that Zebra syncs and validates from an empty state to the chain tip
  test-empty-to-tip-sync:
    name: Test sync from an empty state to the chain tip

Review

@gustavovalverde can review this PR.

Reviewer Checklist

• Config and comments make sense

Follow Up Work

Add the full sync test:

• Run full sync tests on Mainnet #1592