* tests/sanity/80columns.test, tests/sanity/includes.test,
tests/sanity/private.test, tests/sanity/style.test: Have each test
record failure in a separate file.
* python/spot/aux.py: Catch errors from dot and signal them.
* tests/python/_aux.ipynb: New file.
* tests/Makefile.am: Add it.
* tests/sanity/ipynb.pl: Support the convention that tests starting with
'_' should not be published on the web site.
