test: test open channel with unknown fees

[enaples]

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Additional info

To run the test: EXPERIMENTAL_DUAL_FUND=0 uv run python -m pytest -vvvv tests/test_opening.py::test_opening_with_unknown_feerates

Note that in the same node configuration (dev-no-fake-fees=true):

1. if the experimental-dual-fund is enabled, the channel will open only if feerate is specified in the fundchannel call (test at tests: test reproduce #8847 #8864 ), otherwise it will raise

{'code': 501, 'message': 'Cannot estimate fees (yet)'}

2. if f the experimental-dual-fund is NOT enabled, the channel funding will fail both if the feerate is specified or not.

[rustyrussell]

Thanks for the test case!

I would love you to enhance this a little:

1. Use with pytest.raises(RpcError, match='xxxx') to check that the error is the one you expect.
2. Test this with no feerate specified (should fail), then check that it does work with feerate=
3. Mark the test with @pytest.mark.xfail(strict=True).

Then I can write the fix on top of this, and remove the xfail mark, as a separate commit!

[rustyrussell]

Actually, opts=opts means "use these options for all nodes". But I think the open will fail if l2 also doesn't know the fees, so you may want opts=[opts, {}] ?

[enaples]

Test logic

In a context in which fees can't be compute, the user needs to specify feerate on the fundchannel call.

1. The RPC fundchannel call should fail if feerate is not specified
2. The RPC fundchannel call should work fine if feerate is specified

Additional Info

I can't use line_graph since it automatically opens a channel which fails due to unknown feerate. That's why I wasn't able to capture the error with RpcError in the first version of the test. The error actually raised during line_graph call and not from the 'external' fundchannel call.