cmd/coordinator: support skipping redundant tests on try runs

Also, don't start obtaining test sharding buildlets early if they're
reverse buildlets. Reverse buildlets are either immediately available,
or they're not. No point monopolizing them earlier than needed.

Updates golang/go#17104

Change-Id: If5a0bbd0c59b55750adfeeaa8d0f81cdbcc8ad48
Reviewed-on: https://go-review.googlesource.com/29473
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
1 file changed