dtrace tests: Run ksh with -p

In particular, avoid loading the user's .profile file, since that can
have undesirable side effects. Most tests were already careful to do
this.

MFC after: 1 week

Fix tests use /etc/motd after r350184 by using an always existing file

Sponsored by: The FreeBSD Foundation

Add a simple script which calls open(2) and others to generate events for
testing.

This test times-out on a quiet system because there is nobody triggers
syscall::open:entry or syscall::: probe while test execution.

Reviewed by: gnn, markj (earlier version)
Differential Revision: https://reviews.freebsd.org/D11671