Load extraLibraryPaths before standard libraries in test suite

[ppenenko]

Summary

Change the library loading order in MaterialXTest to load extra libraries before standard libraries, matching the behavior of MaterialXView and MaterialXGraphEditor where --library paths take precedence.

This allows extraLibraryPaths to provide override implementations for standard MaterialX nodes. NodeDef::getImplementation() returns the first matching target based on document insertion order, so loading overrides first ensures they are selected.

Changes

• Override-first loading: Load extraLibraryPaths before mx::loadLibraries({ "libraries" }, ...) in loadDependentLibraries()
• Source code search paths: Register extraLibraryPaths and libraries/pbrlib/genglsl as source code search paths so that override implementations can #include standard library source files

Motivation

MaterialXView already loads user-specified libraries first:

// Append the standard library folder, giving it a lower precedence than user-supplied libraries.
libraryFolders.push_back("libraries");

The test suite should have the same behavior to enable testing of custom node implementations that override standard nodes.

Test plan

• Existing MaterialXTest render tests pass
• CI passes