Update grouping of fixtures to clarify tasks

[bgins]

Summary

Problem

The current fixtures mix validation and capability checks into verify and refute tasks.

Impact

The task intention for the fixtures is confounded and would be clearer if broken into more task categories.

Solution

Break the fixtures into groups like

1. validate (the UCAN is valid)
2. prove (the UCAN grants some capability with respect to some issuer)
3. refute (the UCAN is invalid)
4. disprove (the UCAN does not grant some capability with respect to some issuer)

Names not final! But the idea is that we would separate out tests that require a check for capabilities, and provide extra context like which capabilities should be available and a timestamp when they should be available.

Additionally, we may be able to add build fixtures where an implementation creates proofs and UCAN, puts the proofs in a store, then checks that their UCAN satisfies some capability check. This would replace the flawed build fixtures referenced in #14.