test: add known broken test for indexing html

'quite' on IRC reported that notmuch new was grinding to a halt during
initial indexing, and we eventually narrowed the problem down to some
html parts with large embedded images. These cause the number of terms
added to the Xapian database to explode (the first 400 messages
generated 4.6M unique terms), and of course the resulting terms are
not much use for searching.

The second test is sanity check for any "improved" indexing of HTML.