net: don't fail test if splice fails because pipe2 is missing

net: don't fail test if splice fails because pipe2 is missing

    This reportedly happens on CentOS 5.11.  The real code will work fine;
    this test is assuming that the unexported slice function will handle
    the splice, but if pipe2 does not work then it doesn't.  The relevant
    code in internal/poll/splice_linux.go says "Falling back to pipe is
    possible, but prior to 2.6.29 splice returns -EAGAIN instead of 0 when
    the connection is closed."

    Reviewed-on: https://go-review.googlesource.com/138838

From-SVN: r264793