ping: Avoid reporting negative time statistics

[jlduran_gmail.com]

Summary

Display a warning when the time goes back.

PR: 192417

Test Plan

• Adapt the tests to also redact stderr

In the event that this revision is not accepted, the current behavior of printing negative time statistics will be tested as well.

[markj]

sbin/ping/ping.c

1219

I'm not sure how I'd interpret "taking countermeasures" as a user. I think the message should either describe what the code is doing ("clamping time to 0"?) or not say anything in particular.

[jlduran_gmail.com]

sbin/ping/ping.c

1219

Yes, I do not like that message either. I chose it because there's already a precedent with iputils.
Not saying anything, somehow feels misleading.

[jlduran_gmail.com]

Address suggestions:

• Change the warning message

I can submit a different approach, if preferred, where a negative round trip time is displayed as usual, but the final population standard deviation won't calculate the square root of a negative number. To me, the idea of a negative round trip time is warped, that's why I still prefer this revision.