Abstract away Tunnel logic and return real errors

[m-chrzan]

Expected Behavior

The ReleaseGold unit tests, which use a "mock tunnel" interface for calling the initialize function, should test for the exact expected error messages.

This can be done with recoverErrorString, introduced in #11263.

Additionally, logic common to tunnels should be abstracted away into a separate contract to inherit from.

Current Behavior

MockInitialize reverts with "unsuccessful tunnel call", not the original error message.

Logic is duplicated between the two tunnel contracts.