Re-introduce integration tests for the node

In the past the node and the wallet were in the same repo. When the wallet was extracted into a separate repo, integration tests were moved to - https://github.com/mimblewimble/grin-wallet/tree/master/integration It would be beneficial to provide a simple way to run integration suits as part of node tests too. There are several possible options (there are some other for sure):

• Move part of the tests to grin repo. Many tests requires wallet functionality too, we could move only tests which don't require wallet, but it doesn't sound great. We could create some mock wallet, not sure if this effort is justified.
• Extract test into another repo and add to grin and grin-wallet as submodule. The issue is how to make cargo happy in this case and how to specify grin-wallet dependency in grin and vice versa.
• Don't touch anything and always run wallet test with local version of grin repo. It adds some time to CI build and UX is poor.