test-sd-device: limit the number of iterations when testing device parent/child functions

test-sd-device: limit the number of iterations when testing device parent/child functions

The test "hangs" and times out on some arm64 machines. It actually works as
expected, but the machine has 2016 children under /sys/devices/system/memory/,
and the tests do a double loop over this, which is slow enough to hit the 120 s
limit. Add a limit on the number of iterations.

Another option would be to exclude "memory" subsystem. But we may have other
subsystems which have the same problem in the future, so I think it'll be more
robust to not try to limit the fix to a specific subsystem.