binman: Test replacing non-section entries in FIT subsections

A previous patch fixes binman to correctly extract FIT subentries. This
makes it easier to test replacing these entries as we can write tests
using an existing helper function that relies on extracting the replaced
entry.

Add tests that replace leaf entries in FIT subsections with data of
various sizes. Replacing the subsections or the whole FIT section does
not work yet due to the section contents being re-built from unreplaced
subentries' data.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>