pwait: Don't use init as a target

The time_unit test case uses PID 1 as a target for pwait. This doesn't
work in a jail. Since all we need is a process that we know won't die
while the test is running, we may as well use ourselves.

MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.
Reviewed by: ngie
Differential Revision: https://reviews.freebsd.org/D58418