Clean up test server implementations

[abrokenjester]

The test rig currently has at least four separate implementations of a Jetty-based test server:

• org.eclipse.rdf4j.http.server.TestServer (in rdf4j-http-server unit tests)
• org.eclipse.rdf4j.repository.http.HTTPMemServer (in rdf4j-repository-compliance)
• org.eclipse.rdf4j.query.parser.sparql.SPARQLEmbeddedServer (in rdf4j-sparql-compliance)
• org.eclipse.rdf4j.federated.server.SPARQLEmbeddedServer (in rdf4j-tools-federation)

All four do essentially the same thing: spin up an RDF4J Server on localhost with a few test repositories to run tests over HTTP against. We should unify these into a single reusable test utility.

[barthanssens]

Seems to be duplicate of #2933

[abrokenjester]

Seems to be duplicate of #2933

Oops! So it is, well spotted. The description on this issue is more up to date so I'll close the other one.

[erikgb]

@abrokenjester @barthanssens WDYT about adopting something like https://mjeanroy.dev/junit-servers/ instead of building our own test utility? I did a small POC, and seems to work well.

[erikgb]

Here is an MVP POC showing how this could look like: #4437