Reland "[ci] New script to generate test reports as Buildkite Annotations (llvm#113447)"

This reverts commit 8a1ca6c.

I have fixed 2 things:
* The report is now sent by stdin so we do not hit the limit on the size
  of command line arguments.
* The report is limited to 1MB in size and if we exceed that we fall back
  to listing only the totals with a note telling you to check the full log.

Author: DavidSpickett