Add depth tests confirming the existing DTD machinery handles parameter
entities within element-model and attribute-list declarations, deep (5-level)
PE composition, and an external subset that itself loads a further external
parameter entity through the resolver. The Tier-3 DTD PE / external-subset
depth item was already implemented; these lock it to full depth. (The XML
1.0 PE-reference spacing rule, leading/trailing space on DTD-content
inclusion, remains a known minor non-conformance.)
Part of #105 (Tier 3)
