AppVeyor: Add test

.appveyor.yml

@@ -0,0 +1,17 @@
+image:
+- Visual Studio 2019
+platform:
+- x64
+configuration:
+- Release
+cache:
+- C:\Tools\vcpkg
+install:
+- git -C C:\Tools\vcpkg pull --quiet
+- C:\Tools\vcpkg\bootstrap-vcpkg
+- vcpkg --triplet x64-windows install ffmpeg libepoxy libpng libzip sdl2 sqlite3
+before_build:
+- cmake . -DCMAKE_PREFIX_PATH=C:\Qt\5.12\msvc2017_64 -DCMAKE_TOOLCHAIN_FILE=C:\Tools\vcpkg\scripts\buildsystems\vcpkg.cmake
+build:
+  parallel: true
+  project: mGBA.sln