test/unit/local/index: Factorize stream creation
test/unit/local/index: Factorize stream creation

  To avoid having to download an actual file when testing the local
  writer, we stub the `Writer.other` attribute with an object offering
  the `createReadStreamAsync()` method.

  We had quite a lot of duplication within the test file around the
  creation of this object and we introduce here a function that returns
  an object that fits the needs of each example.

cozy-labs/cozy-desktop,cozy-labs/cozy-desktop,cozy-labs/cozy-desktop,cozy-labs/cozy-desktop