Remove test cases with missing JSON-LD contexts.

[peacekeeper]

The test cases case-6.json, case-7.json, and case-8.json contain JSON-LD contexts which don't seem to exist:

• https://mavennet.github.io/contexts/bill-of-lading-v1.0.jsonld
• https://mavennet.github.io/contexts/crude-inspection-v1.0.jsonld
• https://mavennet.github.io/contexts/qp-in-bond-v1.0.jsonld

So I think we should either 1. fix those test cases, or 2. restore the missing JSON-LD contexts, or otherwise 3. delete the test cases (= this PR).