Build and test Debug configuration too

appveyor.yml

@@ -2,6 +2,9 @@ before_build:
   - cmake .
 
 build_script:
+  - msbuild tinyxml2.sln /m /p:Configuration=Debug /t:ALL_BUILD
   - msbuild tinyxml2.sln /m /p:Configuration=Release /t:ALL_BUILD
-  - cd Release
+  - cd %APPVEYOR_BUILD_FOLDER%\Debug
+  - xmltest.exe
+  - cd %APPVEYOR_BUILD_FOLDER%\Release
   - xmltest.exe