vbe: Add a test for VBE device tree fixups

When a FIT includes some OS requests, U-Boot should process these and add
the requested info to corresponding subnodes of the /chosen node. Add a
pytest for this, which sets up the FIT, runs bootm and then uses a C
unit test to check that everything looks OK.

The test needs to run on sandbox_flattree since we don't support
device tree fixups on sandbox (live tree) yet. So enable BOOTMETH_VBE and
disable bootflow_system(), since EFI is not supported on
sandbox_flattree.

Add a link to the initial documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>