Enable the qt backend test on windows

The ```configuration``` is required by the visual studio generator to determine
whether to use a Debug or a Release setup (or a user customized one). It's
required by whatever uses that generator rather then just the GUI code.

Add instructions for the Visual Studio Project File generator to the README
and enable the test on Windows.

backend/tests/README

@@ -8,7 +8,17 @@ ZINT_TEST option enabled:
   mkdir build
   cd build
   cmake -DZINT_TEST=ON ..
-  make
+  cmake --build .
+
+When using generators that support multiple build configurations, such as
+Visual C++ Project Files (the default generator on win32), the configuration
+can be provided via --config:
+
+  cd <project-dir>
+  mkdir build
+  cd build
+  cmake -DZINT_TEST=ON ..
+  cmake --build . --config Debug
   
 ------------------------------------------------------------------------------
 
@@ -29,6 +39,12 @@ To run all tests (within <build-dir>):
 
   ctest
 
+When using a generator that does support multiple build configurations, the
+configuration that was used to build the project has to be explicitly provided
+to ctest, even if it was the default one:
+
+  ctest -C Debug
+
 For various useful options, e.g. matching (-R) and excluding (-E) tests, see
 https://cmake.org/cmake/help/latest/manual/ctest.1.html#options
 

backend_qt/CMakeLists.txt

@@ -21,7 +21,7 @@ target_include_directories(${PROJECT_NAME} PUBLIC "${CMAKE_SOURCE_DIR}/backend")
 
 target_link_libraries(${PROJECT_NAME} zint Qt${QT_VERSION_MAJOR}::Widgets Qt${QT_VERSION_MAJOR}::Gui)
 
-if(ZINT_TEST AND NOT WIN32) # Fails with "Test not available without configuration" on Windows so skip
+if(ZINT_TEST)
     add_subdirectory(tests)
 endif()