tests: opening channel forcing feerate while lightningd unable to est…

[enaples]

…imate fees

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I wrote a test to prove #8852.

Basically, in a situation where lightningd is not able to estimate fees because the mempool is empty, if I try to open a channel forcing the feerate, the fundchannel call fails anyway.

[enaples]

Correct RPC call is:
lightning-cli fundchannel -k "id"=<peer_id> "amount"=<amount> "feerate"='<fee>perkw/perkb'

as referenced in feerates