Moved the function for generating a payload of random byets of a specified
length into the testbench package so that it's availbale for all tests to use.
Added a test case to the IPv4 ID uniqueness test which uses a payload length
of 512 bytes. This test case passes for gVisor currently, whereas the test case
with a small payload of 11 bytes fails because gVisor only assigns the ID field
if the IP payload is sufficiently large.
PiperOrigin-RevId: 316185097
82313667ea