tests: don't wait forever on GNU/Hurd

tests: don't wait forever on GNU/Hurd

* tests/cp/parent-perm-race.sh: Add timeouts so that the test does
not wait forever on GNU/Hurd.  This does not fix the underlying
bug but at least lets the tests make progress.
Problem reported by Nelson H.F. Beebe in: http://bugs.gnu.org/22624
Also, check that 'cp' actually copies the data; this is a better
test anyway, and simplifies the test code.
Also, avoid unlikely race if 'pid' is set in the environment.