erp5testnode: Use slapos.cookbook and erp5.util in develop mode
Like in ERP5 SR (see 09af96ef and 52476645) let's hook slapos recipes & erp5.util codes in erp5testnode SR tightly.
The reason is erp5testnode SR uses both and when one is working on erp5testnode-related changes in both eggs it either creates management overhead to release both eggs to pypi, or is simply hard for people not that handy with slapos to hook things into development mode locally.
@rafael, if we apply this patch you'll need to care about pinning erp5 & slapos.cookbook also in testnode. Thus I suggest to move the pins from leaf users close to original section definitions.