validation: establish a pattern for metatests

[geoknee]

This diff is actually very small, it only seems big because of a rename.

The philosophy here is to place test helper code in files which do not have _test.go suffix. Only the top level entry file needs to have that suffix (validation_test.go). This frees up the associated _test.go file to hold metatests: namely tests which try to ensure the test code itself is bug free.

I added one such new metatest which exposes a bug in our version checking code. You can see this by the failing CI check.

This PR also includes the fix to make that test pass.

Towards #686

[geoknee]

The idea would be to have the majority of the complexity of our validation tests refactored into functions which are:

• pure
• do not take a testing.T (are not tightly coupled to a testing framework)
• do not do any data scraping, but accept the "on chain data" as input
• return useful information about whether the input data passes validation

This way, we can easily:

• inject test data locally
• reuse the pure functions in other applications easily.
• write "meta tests" (we wouldn't even need to call them that) easily without having to mock testing.T.