itest: add a PoC for refilling channel balance

[yyforyongyu]

Suppose a route Alice -> Bob -> Carol. Bob doesn't have enough outgoing liquidity to forward a payment sent from Alice to Carol. This PR adds an itest to show that, with the help of htlc interceptor, Bob can make the payment when his outgoing channel is refilled. The test setup,

1. Channel setup: Alice -> Bob -> Carol.
2. Bob sends payment to Carol, so his outgoing liquidity is drained.
3. Alice sends a payment to Carol via Bob, with Bob intercepting the HTLC and putting the payment on hold.
4. Carol sends payment to Bob, which refills Bob's outgoing liquidity.
5. Bob releases the HTLC.
6. Alice's payment is now succeeded.

[github-actions]

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[coveralls]

Pull Request Test Coverage Report for Build 9701570315

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 170 unchanged lines in 24 files lost coverage.
• Overall coverage decreased (-0.06%) to 58.386%

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Files with Coverage Reduction	New Missed Lines	%
sweep/fee_bumper.go	1	93.12%
log.go	2	93.2%
discovery/sync_manager.go	2	91.2%
chainntnfs/btcdnotify/btcd.go	2	82.78%
htlcswitch/switch.go	2	82.29%
lnrpc/lightning_grpc.pb.go	2	36.58%
lnwallet/rpcwallet/rpcwallet.go	3	72.02%
contractcourt/breach_resolver.go	3	88.33%
peer/ping_manager.go	3	84.85%
lnwallet/rpcwallet/healthcheck.go	3	58.82%

Totals
Change from base Build 9681515217:	-0.06%
Covered Lines:	123312
Relevant Lines:	211201

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💛 - Coveralls