Add several missing APC and XCache tests

  * db4f9ae: because APC and XCache cannot be loaded together, add XCache emulation via APC so that we can test XCache backends;
  * 81e6f27: add tests of APC and XCache adapters to AnnotationsAdaptersTest (only of APC and XCache present; XCache emulation may be used)
  * b5ff26d: add test of XCache adapter to ModelsMetadataAdaptersTest