tests: read limited amount from /dev/urandom

tests: read limited amount from /dev/urandom

In some test environments like e.g. Ubuntu & Debian autopkgtest it
can happen that while generating random device names the pipes
between tr and head are considered dead while processing.
That prints (non fatal) issues like:
  Running ip/link/new_link.t [iproute2-this/4.13.0-17-generic]: tr:
write error: Broken pipe
  tr: write error
  PASS

This only happens if reading an infinite amount of chars with the
read from urandom, so reading a defined amount fixes the issue.

Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>