What problem does this feature solve?

In the early stages of Rspack, we built a diff test to compare the output code of Rspack and webpack.

Over time, this test became less usable, until we plan to remove it from the Rspack repository to speed up CI. See #8270.

In the future, we can still build CI in a separate repository to compare the output code of different versions of Rspack and webpack. This issue is to record this goal.