dtoc: Update tests to write failures to /tmp

dtoc: Update tests to write failures to /tmp

When a test fails due to an output mismatch (e.g. due to a new property
being adding to a test file) it is currently hard to update the test to
the new output. In particular the tabs in the file are written as \t in
the Python tests.

To make this easier, write both the expected and actual results to /tmp
to allow use of meld, and copying into the test.

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