Wrote MaterialPackageWrangler.open_illustration_pdfs() tests.

Description: use (pdfo) to open the illustration.pdf file in each visible
material package.

Example:

    Red Example Score (2013) - materials

       1: instrumentation
       2: magic numbers
       3: pitch range inventory
       4: tempo inventory

          materials - copy (cp)
          materials - new (new)
          materials - remove (rm)
          materials - rename (ren)

    > pdfo

    Will open ...

       /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/materials/pitch_range_inventory/illustration.pdf
       /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/materials/tempo_inventory/illustration.pdf

    Ok? y

    >

MAPPING: this means it is now possible to open the illustration.pdf files for
all material packages ever created with a six keystrokes: (m pdfo).