test: mbr: Use a constant for the block size

test: mbr: Use a constant for the block size

It isn't that important to factor out constants in tests, but in this
case we have 0x200 and 512 used. The commands don't use the constant
as they use a block count ('1'). It doesn't create more code to use a
constant, so create one.

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