validation: add chain config negative testing

[geoknee]

Much or our validation code is untested, which means we are very prone to bugs like this #685.

We should be using local dummy data to ensure validation checks catch chains with non-standard or missing data. We should refactor tests as necessary to enable them to be tested in this way.

[geoknee]

One area that seems particularly brittle is the parsing of the standard TOML files. Coupled with the approach we take to ignore empty structs in several places, small typos break things easily. See #705.

[geoknee]

The direction I would like us to take on this is to:

• move as much of the complexity as possible into pure functions which live in regular .go files (NOT _test.go files)
• this formalizes that code as production code, not test code
• this makes the code easy to test without putting fake data on chain
• we add codecov to this repo and start ramping up the coverage of this newly ordained "production code' by writing tests for it