test: sf: Add a simple SPI flash test

test: sf: Add a simple SPI flash test

The current test is a functional test, covering all the way from the
command line to the sandbox SPI driver. This is useful, but it is easier
to diagnose failures with a smaller test.

Add a simple test which reads and writes data and checks that it is stored
and retrieved correctly.

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