binman: Make the operation of Entry__testing explicit

binman: Make the operation of Entry__testing explicit

This fake entry is used for testing. At present it only has one behaviour
which is to return an invalid set of entry positions, to cause an error.

The fake entry will need to be used for other things too. Allow the test
.dts file to specify the behaviour of the fake entry, so we can control
its behaviour easily.

While we are here, drop the ReadContents() method, since this only applies
to subclasses of Entry_blob, which Entry__testing is not.

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