bug-fix test-driving input

The client driver is a little restrictive in the escape sequences it
handles; two octets here were missing the `x` after the `\`, so `\05` is
two octets, a 0 and then a 5, where `\x05` would be one octet.

So we were sending two more octets than expected, not catching that Exim
was parsing the wrong IP/port at the end, and now that Exim only reads
as much of the proxy protocol header as belongs in it, instead of "up to
the largest it could be", this test-bug has been exposed.