Alleviate CMake target name clashes, visibility, and grouping.

Simplify <platform>_generate cmake function signature, as TARGET_ALIAS and TARGET_NAME are still in scope and do not need to be passed.
Add USE_FOLDERS, and FOLDER properties to group targets in VS and XCode
Guard test target with GODOT_ENABLE_TESTING
Generate all three variants in the form godot-cpp.test.<target> to remove the -DTEST_TARGET option clutter.
Update the docs/cmake.rst with information about the testing target
Update the Github CI

cmake/android.cmake

@@ -29,13 +29,12 @@ function( android_options )
     # Android Options
 endfunction()
 
-function( android_generate TARGET_NAME )
-
+function( android_generate )
     target_compile_definitions(${TARGET_NAME}
             PUBLIC
             ANDROID_ENABLED
             UNIX_ENABLED
     )
 
-    common_compiler_flags( ${TARGET_NAME} )
+    common_compiler_flags()
 endfunction()