Restore `CTK_USE_QTTESTING` support with Qt 6

[jcfr]

Is your feature request related to a problem? Please describe.

CTK_USE_QTTESTING is currently disabled for Qt 6 builds because the CTK-side QtTesting integration depends on Qt XmlPatterns, which is not available in Qt 6. As a result:

• Projects building CTK with Qt 6 cannot enable QtTesting-based recording/replay and regression tests.

• CTK’s QtTesting integration does not reflect the actual upstream dependencies:

  • Qt XmlPatterns C++ classes are only used by ctkXMLEventSource in CTK/Libs/QtTesting.
  • Other XmlPatterns usages are limited to components such as CTK/Libs/CommandLineModules and CTK/Applications/ctkCommandLineModuleExplorer, which are not required by all CTK consumers.
  • The upstream QtTesting library (commontk/QtTesting¹) itself does not depend on Qt XmlPatterns.

• This creates an unnecessary limitation for Qt 6 adopters who rely on QtTesting workflows.

Describe the solution you'd like

Make CTK_USE_QTTESTING=ON supported when building with Qt 6 by:

• Aligning CTK’s QtTesting integration with the upstream QtTesting library so it no longer requires Qt XmlPatterns for core functionality.

• Refactoring or isolating ctkXMLEventSource and any XmlPatterns-based logic so that:

  • XmlPatterns is only used in Qt 5 builds (if needed), or
  • XmlPatterns-dependent code is replaced with Qt 6–compatible XML handling.

• Updating the CMake configuration so that:

  • CTK_USE_QTTESTING is not forcibly disabled for Qt 6 once dependencies are addressed.
  • Appropriate Qt 6 modules (e.g., Qt6::Test) are used when QtTesting is enabled.

• Preserving behavior and configuration for existing Qt 5 users.

Describe alternatives you've considered

• Keeping CTK_USE_QTTESTING permanently disabled for Qt 6:
  • Simple, but prevents users from leveraging QtTesting with modern Qt versions.

Additional context

• CTK now supports building with Qt 5 or Qt 6 via CTK_QT_VERSION.
  • Add support for Qt 6 alongside Qt 5 #1309
• As part of Qt 6 enablement, CTK_USE_QTTESTING is currently forced OFF for Qt 6 to avoid unsupported Qt modules.
• Prior work in Slicer already narrowed XmlPatterns usage to QtTesting-specific paths and optional components, indicating that a clean separation is feasible.
  • COMP: Require Qt XmlPatterns only when QtTesting is enabled Slicer/Slicer#8835
• This feature would restore QtTesting-based regression and UI testing workflows for projects migrating to Qt 6 while maintaining compatibility with existing Qt 5 setups.

Footnotes

1. https://github.com/commontk/QtTesting ↩