[SYCL] Re-enable unit-tests for virtual functions (#15846)

They were disabled because some helpers were missing in UR mock
infrastructure. However, they can be built just fine, meaning that the
mock infrastructure gap has been already closed.

sycl/unittests/Extensions/CMakeLists.txt

@@ -21,6 +21,4 @@ add_sycl_unittest(ExtensionsTests OBJECT
 )
 
 add_subdirectory(CommandGraph)
-
-# Disabled pending UR fix adding setDataAs to mock dummy handle.
-#add_subdirectory(VirtualFunctions)
+add_subdirectory(VirtualFunctions)