In-memory File for unittesting

I'd like to write a unit-test for a method that prints to the standard output. I have already changed the code so it prints to a passed-in File instance instead that is stdout by default. The only thing I am missing is some in-memory File instance that I could pass-in. Is there such a thing? Any recommendation? I wish something like this worked:

import std.stdio;

void greet(File f = stdout) {
    f.writeln("hello!");
}

unittest {
    greet(inmemory);
    assert(inmemory.content == "hello!\n")
}

void main() {
    greet();
}

Any other approach for unit-testing code that prints to stdout?