test: Fix typos and tidy up

Fix review comments that were missed at the time. Also explain why we need
to regenerate the device tree for each test.

Reported-by: Teddy Reed <teddy.reed@gmail.com>
Suggested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: f6349c3c (test: Add a README)