Consider failures as intermittent for a push only if they are not likely (nor possible?) regressions of parent pushes

[marco-c]

This should reduce errors in classifying failures as intermittent.

These failures should not appear in any of the three lists ("real", "intermittent", "unknown") because they are actually due to some parent push.

[marco-c]

Note: this should already be the case because a failure is considered intermittent only if a group passed in some tasks and failed in others. If the failure is consistent and due to a parent push, we should never find ourselves in that situation (we'd see tasks failing everywhere).
The problem could only occur when there is a config-specific failure (so a failure that is happening consistently on a given config, but not happening on another config). In this case, we might consider it an intermittent.
Doing #607 would fix that.

We should nevetheless add a few tests to make sure we're actually good here.

[marco-c]

We should also apply a similar logic to remove failures that appear in children pushes and are due to children pushes from the "unknown" list.