itests: Add assertions to `testCustomChannels` to increase test coverage

[dstadulis]

Gijs and Roas can enumerate additional test cases from their conversation:
testCustomChannels can have additiona assertions added to increase test coverage

Preliminary testing ideas to inspire ideas:

• Assert that asset inputs meet stipulations in Limit number of HTLCs in custom channel taproot-assets#1132
• HTLC errors: forwarding etc
• Partial shard fulfillment
• RFQ violations: client side creates a violation, Inconsistency between rate during quote and forwarding (Edge node updates their rate after issuing quote)
• RFQ tolerance passes within amount or outside of amount
• very small or large amounts payment amounts, attempts to drain sats balance of channel after asset-channel created
• Explore any non standard amount / behavior
• Lost HTLCs due to node dying mid routing process

[gijswijs]

There have been quite a few addition to increase test coverage wrt custom channels. @dstadulis I think we should archive this issue.

[dstadulis]

After CLI forwarding checks are added, Let's close this issue.

[dstadulis]

Given in situ code, this issue has been satisfied.