devtools: update "presubmit poll", "presubmit test" and "dashboard" to support running presubmit tests with multiple parts.
devtools: update "presubmit poll", "presubmit test" and "dashboard" to support running presubmit tests with multiple parts.

- Change the storage dir structure for presubmit results to:
  <os>/<arch>/<test>/<partIndex>
- Aggregate results for all the parts of a test before showing result summary.
  The purpose is to hide "test parts" from end users.
- Similarly, aggregate results/failed tests/console logs for all the parts of a
  test before showing job summary/job in dashboard.

I suggest the following review order:

1. internal/...
2. presubmit/query
3. presubmit/test
4. presubmit/result
5. dashboard

Change-Id: I9b29296aebd33378f2a187e018e7dfb7b689ce88

vanadium/go.jiri,vanadium/go.jiri