binman: Clean up all output directories in tests

At present some tests leave behind output directories. This happens
because some tests call binman, which sets up an output directory, then
call it again, which sets up another output directory and leaves the
original one behind.

Fix this by using a separate temporary directory when binman is called
twice, or by manually removing the output directory.

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