Remove/Rename ibctesting.FirstChannelID

[bznein]

ibctesting.FirstChannelID used to be a valid constant to represent channels used in testing. However, after #6748, this is not true anymore.

We should revisit all occurrences of this constant and either:

• Replace them with the appropriate channel
• Replace them with a properly named constant (most time they can either represent invalid channels or placeholders)

[srdtrk]

Please do not remove this constant from the ibctesting package as I'm using it in external crates. Maybe the issue should be renamed to Remove/Revisit the usage of ibctesting.FirstChannelID since the constant is appropriately named?

[chatton]

Yeah I think keeping the constant in place is doing no harm to not mess with external dependencies on it. As long as our tests are updated to not rely on it, I think it's fine to keep.

[DimitrisJim]

yup, I think just a better name can be introduced, MockChannelID, InvalidChannelID etc, defo no need to rm esp if one of our team is even using it!