Exporting TestEndpoints

[aljones15]

This library contains a Class TestEndpoints https://github.com/w3c/vc-data-model-2.0-test-suite/blob/main/tests/TestEndpoints.js

This class has a number of useful features:

1. it groups issuers, verifiers, and vpVerifiers around a tag
2. The later allows for mixing issuers and verifiers for a single normative statement

However the class needs some clean up

1. methods that make http calls should return the result from implementations
2. The post function in the class should be taken out (all calls should go through the vc-test-suite-implementations package)
3. This snippet can be removed
4. receive-json.js can be removed

The following is a related issue to how this suite differs from other suites: #4