binman: Correct pylint errors

Fix pylint errors that can be fixed and mask those that seem to be
incorrect.

A complication with binman is that it tries to avoid importing libfdt
(or anything that imports it) unless needed, so that things like help
still work if it is missing.

Note that two tests are duplicated in binman and two others have
duplicate names, so both of these issues are fixed also.

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