Messaging OSS Testapp Prep

Re-organized Messaging test files into tests/ folder, and added CMakeLists.txt file for tests.
None of the messaging tests run on desktop, so no tests are actually set up yet (pending ios and android tests being ready for OSS).

PiperOrigin-RevId: 259568448

Author: ryanmeier

messaging/CMakeLists.txt

@@ -138,3 +138,8 @@ if(IOS)
   # Add a dependency to downloading the headers onto messaging.
   add_dependencies(firebase_messaging ${pod_target_name})
 endif()
+
+if(FIREBASE_CPP_BUILD_TESTS)
+  # Add the tests subdirectory
+  add_subdirectory(tests)
+endif()