I had previously tried on a big EPUB. Trying the testplugin on the empty default EPUB as you suggested gives a success.
Code:
Statut :success
Verify sys.path settings manually
/usr/share/sigil/plugin_launchers/python
/usr/lib/python35.zip
/usr/lib/python3.5
/usr/lib/python3.5/plat-linux
/usr/lib/python3.5/lib-dynload
/usr/lib/python3.5/site-packages
/home/roger/.local/share/sigil-ebook/sigil/plugins/testplugin
Verifying proper Python packages are available
Python Package: PIL Found
Python Package: cssselect Found
Python Package: cssutils Found
Python Package: html5lib Found
Python Package: lxml Found
Python Package: regex Found
Python Package: chardet Found
Python Package: six Found
Verifying Sigil Python Libraries can be found/loaded
Sigil Python library: epub_utils Found
Sigil Python library: quickparser Found
Sigil Python library: compatibility_utils Found
Sigil Python library: sigil_bs4 Found
Verifying Hunspell Spell Checking
Hunspell en_US affix file and dictionary Found
Hunspell shared library Found
Hunspell spellchecking works True
Verifying Sigil Gumbo Library operation
Sigil Gumbo BS4 Adapter library Found
Sigil BS4/Gumbo Parser works True
Verifying selected basic plugin interface elements
bk.text_iter() works True
bk.manifest_iter() works True
bk.spine_iter() works True
bk.other_iter() works True
reading a non-manifest book file works True
reading a specific OPF manifest id works True
writing to a specific OPF manifest id works True
Launcher Version 20160909
ePub Version 3.0
adding your own file to the manifest works True
basename to id mapping works True
deleting our added file works True
Success - All Tests of Plugin Operations Passed