sysv test: properly wait for children

In the msg and shm tests, if the child exited before the parent
entered sigsuspend(), the test would hang and time out. This was
also a problem in the sem test, but the misuse of atf_tc_pass()
masked it. Adding a short sleep before the sigsuspend() calls made
the hang 100% reliable. With the same sleep in the new version,
the test passes reliably.

Remove calls to atf_tc_pass(). The call in the sem test broke the test
by exiting prematurely, after only one child out of five had finished.
The other two were harmless but unhelpful.

Reduce a one-second sleep to a more reasonable duration so I can quickly
run many iterations of the test.

Where feasible, assert that wait() returns the child PID. While I'm here,
use the more succinct ATF_REQUIRE* instead of if/atf_tc_fail/else.

Flush stdout before forking to avoid double-flush.

Use errx() when errno is irrelevant.

Don't use ATF_REQUIRE* in children. Apparently, the output doesn't
get saved. The exit status works, so it fails correctly, but silently.

Re-enable the test in CI.

PR: 233649
Reviewed by: markj (previous version)
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D35187