Print top N slowest targets... (#1989)

...when --print_shard_summary is set.

This change adds a new annotation that shows the 20 slowest
test(shard)s.

Example:
https://buildkite.com/bazel/bazel-bazel-macos-ninja/builds/1939#annotation-slowest_targets
(Note: The latest version of the code drops the "in 1 out of 1 run(s)"
part.)
1 file changed