Do not use this function in production code. For use in **unit tests**.
This function initialises a new File with metadata as though the
File has been retrieved from the given URL. The mock File can be used in
unit tests that require persisted Files; e.g. unit tests that call [[getSourceUrl]].
This function initialises a new File with metadata as though the File has been retrieved from the given URL. The mock File can be used in unit tests that require persisted Files; e.g. unit tests that call [[getSourceUrl]].