Cleaner context mocks for ICS18 and ICS26

[adizere]

Summary

The last bits as part of overhauling mock context cosmos/ibc-rs#111: this issue addresses specifically the mock context for ICS18 and ICS26. Once we clean these up, we will also be able to remove the old context mocks (MockConnectionContext, MockClientContext)

@ancazamfir raised two interesting comments while reviewing the prior iteration of work on mock contexts:

Wondering if we should check for sanity for the context, even if it's mock, just to make sure the tests don't initialize with a bad context. For example check that the client of a connection is present. Could also add the connection to client here.
I am also wondering if we should allow for a vector of connections, i.e. call it with_connections().

TODOs:

• Anca's two suggestions
• modules/src/ics03_connection/handler.rs the context should not be mutable

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