Add tests, improve coverage

[ManuDeBuck]

Add extra tests to lift up the coverage. A special case that should we should look in to is the public async setUpServer() : Promise<void> function in the LdfResponseMocker.ts file.

Also add extra tests for HDT- and RDFJS datasource testing.

[ManuDeBuck]

Have a look at #26 for a fun testcase!