After some fiddling I actually managed to solve the problem.
Here's the code:
//this Importer is used to create mockTracks
std::shared_ptr<MockImporter> datImporter = std::make_shared<MockImporter>();
EXPECT_CALL(*(datImporter.get()), parseSource()).Times(3);
EXPECT_CALL(*(datImporter.get()), getSource()).Times(3).WillRepeatedly(Return(mockedTrack()));
//this factory is used to inject the actual MockImporter
std::shared_ptr<MockImporterFactory> importerFactory = std::make_shared<MockImporterFactory>();
EXPECT_CALL(*(importerFactory.get()), createImporter(QString("Test1.DAT"))).WillOnce(Return(datImporter));
EXPECT_CALL(*(importerFactory.get()), createImporter(QString("Test2.DAT"))).WillOnce(Return(datImporter));
EXPECT_CALL(*(importerFactory.get()), createImporter(QString("Test3.DAT"))).WillOnce(Return(datImporter));
//this injects the MockImporterFactory and runs the code.
TrackLoaderPtr trackLoader = std::make_shared<TrackLoader>(importerFactory);
trackLoader->loadTracks(identNode(), _dataManager);
Basically, ImporterFactory was first mocked to ensure that it creates any Importer at all.
By adding the .WillOnce(Return(datImporter)) bit, I was able to inject a MockImporter through the MockImporterFactory.