Better re-use test code between sections

[peacekeeper]

After merging #86, all the statements in DID Resolution and DID URL Dereferencing will have tests, but there is still some duplication.

E.g. a test from the DID Resolution section like the one in #86 (review) should re-use test code from the Metadata Structure section (see https://github.com/w3c/did-test-suite/blob/main/packages/did-core-test-server/suites/did-core-properties/did-metadata-structure.js).