XD-1838 FileSourceTest applies uses labels

* Currently Acceptance FileSource Acceptance Tests are failing
* This is because the sink that tests the result for the file source test is a filesink. Both use the "file" token. Thus causing a failure
* SimpleFileSource and SimpleFileSink needs to support a label method.
* Update testFileSource to use the labels.

This should be moved at AbstractModuleFixture level, as all modules will need this functionality.

Note that you can still retain strong return type for label() by using the `AbstractModuleFixture<F extends AbstractModuleFixture<F>>` trick (and have label() return `<F>`)