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@).
(cherry picked from commit 1032131464f1196ad674b30c14a9e611789a1061)