Print an aggregated summary for sharded tests (#1870)

With a high number of shards it becomes very hard to see which tests are
actually failing. This change introduces the `--print_shard_summary`
flag. If set, there will be one Buildkite annotation per failing sharded
platform that contains a test summary of all failing tests.

Example: https://buildkite.com/bazel/bazel-bazel-macos-ninja/builds/420

Related to
#1708