Is there a File-like object for unit tests?

[Amit]

Hi!

I wrote a text parser that takes a File argument and parses that 
file's contents. Now I would like to write a unit-test for that 
parser. I need a File (or a general IO interface) that reads from 
an in-memory buffer, similar to python's `StringIO` or go's 
`strings.Reader`.

How can I achieve that?

Example of what I have in mind:

```d
unittest {
     string fakeContents = "foo\nbar\nbaz";
     File f = createFileFromString(fakeContents);
     assert(myParse(f) == MyParsedObject(...));
}
```