Since we don't have standby nodes anymore, we don't need to close the
channels when the test finishes. Previously we would do so to make sure
the standby nodes have a clean state for the next test case, which is no
longer relevant.
This commit removes the usage of the standby nodes and uses
`CreateSimpleNetwork` when applicable. Also introduces a helper method
`NewNodeWithCoins` to quickly start a node with funds.
The method `AssertActiveHtlcs` is now removed due to it's easy to be
misused. To assert a given htlc, use `AssertOutgoingHTLCActive` and
`AssertIncomingHTLCActive` instead for ensuring the HTLC exists in the
right direction. Although often the case `AssertNumActiveHtlcs` would be
enough as it implicitly checks the forwarding behavior for an
intermediate node by asserting there are always num_payment*2 HTLCs.
