Drives the unmodified httpd.c and uip.c through a handshake and a GET
from a client that moves its receive window, and compares the bytes that
reach the wire with the file they are meant to carry. The served file
holds a position-dependent pattern, so a repeat or a gap is reported at
its offset rather than as content that differs somewhere.
The first scenario holds the window still and must pass: it is the check
that the bench itself is sound, before the two moving-window scenarios
are believed.
