Allow append in legacy cpp info mockup

[jcar87]

Changelog: Bugfix: Fix issue to allow appending to objects returned by the legacy build_modules mock object.

(Similar to #12673)

Fixes issue with pybind11 recipe in Conan Center with Conan2:

      cmake_file = os.path.join(cmake_base_path, "pybind11Common.cmake")
        self.cpp_info.set_property("cmake_build_modules", [cmake_file])
        for generator in ["cmake_find_package", "cmake_find_package_multi"]:
            self.cpp_info.components["pybind11_"].build_modules[generator].append(cmake_file)