Automerge: Only restart failed libc++ jobs, not cancelled ones. (#146397)

Despite the error message for preempted jobs containing the words
"cancelled", these are considered workflow "failures" by github.

This is important, because if we fail to distinguish between "failed"
and "cancelled" jobs, the restarter will fight to restart jobs a user
intentionally cancelled (either by pressing the "cancel" button, or by
pushing an update to a PR).

This reverts commit 3ea7fc7. This also
reverts earlier attempts to solve this problem by matching the messages
to detect manual cancellations.

This change also removes ldionne's test workflow, as its hard to
correctly keep in sync.

This change does not attempt to address the maintainability or
testability of this script, which continues to be an issue. If asked to
address these issues, my plan is to write the script in python (which
most people are more familar with), and turn this action into a "docker
action" using a container with the python action and dependencies built
into it. Let me know if that's a direction we're interested in heading.

Author: EricWF