Mock Fusion instance correctly in IndexStageTestBase

[krook1024]

Tl;dr

Mock Fusion instance correctly in IndexStageTestBase

Details

Previously, if the IndexStage you were testing contained a call to IndexStage::newDocument, the unit test would fail because of an NPE because the tested IndexStage environment tries to create a document, but it cannot because the behaviour of the Fusion instance that's injected into the IndexStage isn't mocked correctly.

This PR fixes that behaviour and lets IndexStage instances run newDocuments, but it runs the IndexStageTestBase::newDocument in the background.

How to verify

Let's suppose you have an IndexStage that looks something like this:

public class MyStage extends IndexStageBase<MyStageConfig> {
// [...]
    @Override
    public void process(Document document, Context context, Consumer<Document> output) {
       Document document = newDocument();
       output.accept(document);
    }
// [...]
}

And the test looking like this:

public class MyStageTest extends IndexStageTestBase<MyStageConfig> {
// [...]
 @Test
    public void testWhenValidUrlIsProvidedDocumentShouldBeFed() {
         MyStageConfig conf = newConfig(MyStageConfig.class, config -> {
             // [...]
        }); 

        MyStage underTest = createStage(MyStage.class, conf);

        underTest.process(doc, null, System.out::println);
    }
// [...]
}

It would end up in a NPE in the current version, but with this PR it should print the new document.

[krook1024]

@dzmitryk @sbrowndev can you guys take a look this?