Group targets into folders

This plays nicer with IDEs.
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 79c2684..98cc771 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -59,6 +59,7 @@
     target_link_libraries(${target} PRIVATE ${ARG_LIBS})
     target_link_libraries(${target} PRIVATE gmock_main)
     add_test(NAME spirv-tools-${target} COMMAND ${target})
+    set_property(TARGET ${target} PROPERTY FOLDER "SPIRV-Tools tests")
   endif()
 endfunction()