mpp_split: use single nodes for mpp payments over trampoline

[bitromortac]

Adds the node id as an input to mpp_split (extending the short_channel_id key), to be able to filter out split configurations that pay via several nodes. We need to do this in case of trampoline multipart payments, see ACINQ/eclair#1723. Related: #7087.

TestPeer now inherits from TestCaseForTestnet instead of ElectrumTestCase to disable filtering of single nodes in tests, to not break test_multipart_payment_with_trampoline, which we want to keep in its current form (as intended for the future without the above mentioned bug). Let me know if there's a better way to do this.

[ecdsa]

It looks good. Note that we use self.channel_db is None as a way to detect trampoline all over the place. It may be clearer to use an explicit variable name or method, but in that case it should be done everywhere, not just in a single method. I think introducing heterogeneous notations is worse. Also, if a variable is a boolean, the name should reflect it, eg is_trampoline instead of trampoline_payment.

[bitromortac]

I reverted to not self.channel_db which is also used in other places, to unify. Also a good point concerning naming of boolean variables.