Test variation data

[Hoolean]

We want to test heights at more than the default location of VFs. We should consider how much variation to test, ranging from:

1. Named instances
2. Named instances + product of axis extremes
3. Named instances + product of axis extremes + product of axis locations with variation data

This list of locations could grow very fast, so we should see what is practical with the performance we can achieve.

[Hoolean]

Initially we will just implement (2), as (3) requires us to provide RustyBuzz with pre-normalised coordinates, which it does not have a public API for accepting at the moment.

[Hoolean]

Spun (3) out into #12