tests: net: Offset downloads to 4MB

The network test currently downloads files at 0MB offset of RAM start.
This works for most ARM systems, but x86 has weird memory layout constraints
on the first MB of RAM.

To not get caught into any of these, let's add a 4MB pad from start
of RAM to the default memory offset.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>