Invariant checking during longer-term runs

[kostko]

We should add tests that check if any invariants are being violated during longer runs. The test(s) would run long-term scenarios (injecting various kinds of transactions).

There needs to be:

• A generic way to describe system invariants (e.g., total amount of tokens as accounted from all the places that hold tokens should stay the same) such that it is easy to check them at any point in time. This may be possible to reuse in Write a comprehensive set of auditing tools #1633.
• A generic way to inject transactions (consensus and runtime, with priority on consensus first) during the long-term scenarios. Probably using some kind of random distribution, based on an initial random seed.

The tests can run using our existing test runner, and they should be run daily in CI on the current master.

Estimated cost: ~1 sprint