TST: add mixed-type panels & frames to generate_legacy_pickles

Don't generate them for pre-0.14.1 as they cannot be unpickled.
BUG: fix unpickling of some pre-0.14.1 pickles with non-unique items

Series, frames and panels that contain only one block can be unpickled
under the assumption that block items correspond to manager items 1-to-1.