New location for legacy tests

[tqpcharlie]

In the version of ethereum/tests currently used by Geth, the legacy tests are stored as a folder in the main test repo. In the newest version, they are in a seperate repo (ethereum/legacytests). This actually causes the CI to fail when using the new version, which blocks PRs such as #23570, which relies on the new version of the tests repo.
Version used by Geth:

New version:

I've found the line that is causing this problem, but I don't know how to update it so it's pointing to a submodule and not a folder.

[holiman]

Yes, this is something we must fix. Not urgent, but definitely