Add a test for `calculatenetworkfee` RPC handler for custom witnesses

[AnnaShaleva]

We need a properly constructed test for calculatenetworkfee RPC handler for custom Koblitz-based transaction witnesses. We also need to ensure that fee calculated by this test match precisely the expected value. This test should not use some static transaction blob since it'll be hard to keep it up-to-date. We also need to enable Cockatrice for our testchains to make this test pass.