Unskip the test skipped in #4173 (#7809)

This test was skipped 12 years ago because it was flaky on FreeBSD and
OpenBSD. Since then, Ruby's SIGCHLD handling has been substantially
re-written (mostly by Eric Wong @normalperson in 44fc3d08).

These tests now in fact pass reliably on Ruby master on FreeBSD 13.2 and
OpenBSD 7.3. I stress-tested the test_wait_and_sigchild test on my
laptop by running four copies of the test in a loop on a 8-core VM; both
by itself and also as part of the whole test_process.rb file. I did not
see any failures.

Let's unskip the test and close [#4173] out. I'll keep an eye out on Ruby
CI for any flakes in this file on BSD after this gets merged, but if we
don't see any I'm going to assume 44fc3d08 or related changes around
that time accidently fixed this bug.

It's also probably important to unskip this test so that if another
attempt at removing the special SIGCHLD handling is made (like was
reverted in https://github.com/ruby/ruby/pull/7517), we get signal if
that breaks on FreeBSD/OpenBSD.

[Fixes #4173]

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