Use memory file system when running unit tests

[atn832]

This fixes errors in unit tests introduced in #12 (link):

  FileSystemException: Cannot retrieve length of file, path = 'someimage.png' (OS Error: No such file or directory, errno = 2)
  dart:io                                                                _File.lengthSync
  package:firebase_storage_mocks/src/mock_storage_reference.dart 105:90  MockReference._createFullMetadata
  package:firebase_storage_mocks/src/mock_storage_reference.dart 26:38   MockReference.putFile

Instead of referring to a file on the file system which does not exist, we use the memory file system to set up a file with actual data.