ping: fix test timestamp_origin when tstamprepl is disabled

[olivier]

Summary

The timestamp_origin test sends an ICMP Timestamp Request (ping -Mt) and parses the tso/tsr fields out of the reply.
When the sysctl net.inet.icmp.tstamprepl is 0 (a common hardened default), the kernel silently drops the request, ping receives no reply, and the sed extraction yields an empty $tso. The test then fails inside atf_check test -n "$tso" with the unhelpful message

Test Plan

example:

$ sudo sysctl net.inet.icmp.tstamprepl=0
net.inet.icmp.tstamprepl: 1 -> 0
 $ sudo kyua test sbin/ping/ping_test:timestamp_origin
sbin/ping/ping_test:timestamp_origin  ->  passed  [0.026s]

Results file id is usr_tests.20260528-002128-026180
Results saved to /root/.kyua/store/results.usr_tests.20260528-002128-026180.db

1/1 passed (0 broken, 0 failed, 0 skipped)
$ sudo sysctl net.inet.icmp.tstamprepl
net.inet.icmp.tstamprepl: 0

[maxim]

It looks good. Thank you!