Remove atf_tc_skip calls from ptrace_test

I've run these tests many times in a loop on multiple architectures and
it works reliably for me, maybe it's time to retire these skips?
This also adds an additional waitpid to one of the tests to avoid
a potential race condition (suggested by markj@).

PR: 239397, 244056, 239425, 240510, 220841, 243605
Reviewed By: markj
Differential Revision: https://reviews.freebsd.org/D28888