Added QAbstractItemModelTester to main and proxy model. And fixing errors in functions flags(...), rowCount(...) and columnCount(...).

CMakeLists.txt

@@ -11,6 +11,7 @@ set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 find_package(QT NAMES Qt6 REQUIRED COMPONENTS Core LinguistTools)
 find_package(Qt${QT_VERSION_MAJOR} REQUIRED COMPONENTS Core LinguistTools Gui)
+find_package(Qt${QT_VERSION_MAJOR} REQUIRED COMPONENTS Test)
 
 configure_file(CoreConfig.h.in CoreConfig.h)
 
@@ -36,6 +37,8 @@ add_library(${TARGET_APP} STATIC
   model/generalsortfiltermodel.h model/generalsortfiltermodel.cpp
 )
 
+
+target_link_libraries(GenericCore PRIVATE Qt${QT_VERSION_MAJOR}::Test)
 include_directories(${CMAKE_CURRENT_BINARY_DIR})
 
 target_link_libraries(${TARGET_APP} PRIVATE Qt${QT_VERSION_MAJOR}::Core Qt${QT_VERSION_MAJOR}::Gui)