If we do not want _all_ tests durations, it makes sense to reuse the existing slow test collection and aggregation

Bug: v8:10168
Change-Id: I500acdb799f41ee6f0fb2c57afb95e1e1830b2dd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2064221
Auto-Submit: Liviu Rau <liviurau@chromium.org>
Commit-Queue: Liviu Rau <liviurau@chromium.org>
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Reviewed-by: Tamer Tas <tmrts@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66342}