Add library-style test support to the test schema

[bryantaustin13]

Summary

Extends tests/testSchema.xsd so a <test> can supply a complete CQL
<library> instead of a single <expression>. This is a first, scoped
increment toward #17 (Multi-Library, Multi-Resource Tests) and unblocks #61,
whose overload-matching test needs library-level function declarations.

Changes

• <test> now accepts an <xs:choice> of <expression> or <library>
  (exactly one required — preserves the "every test has something to evaluate"
  invariant).
• New Library complex type: CQL source text with an optional invalid
  attribute (mirrors Expression), so negative library tests
  (invalid="semantic" etc.) are expressible.
• New optional name attribute on Output, identifying which library
  define an expected result describes.

Compatibility & validation

Purely additive — no existing construct changes meaning. Validated with
xmlschema:

• schema loads as a valid XSD,
• Add the overloads test back in #61's CqlOverloadMatching.xml now validates,
• all existing tests/cql/*.xml still validate (16/16).

Relationship to other work

• Part of Proposal for Multi-Library, Multi-Resource Tests #17 (this is the XML-native, single-<library> slice of that broader proposal — not the full JSON-meta/multi-resource format).
• Runner counterpart: cqframework/cql-tests-runner#B
• Unblocks Add the overloads test back in #61