Query

The Query API needs the Context.Database set, and the example below uses DatabaseSwitcher to do so:

[Fact]
public void HowToWorkWithQueryApi()
{
  const string Query = "/sitecore/content/*[@@key = 'home']";

  using (new Sitecore.FakeDb.Db 
    {
      new Sitecore.FakeDb.DbItem("home")
    })
  {
    Sitecore.Data.Items.Item[] result =
      Sitecore.Data.Query.Query.SelectItems(Query);

    Xunit.Assert.Equal(result.Count(), 1);
    Xunit.Assert.Equal(result[0].Key, "home");
  }
}

[Fact]
public void HowToWorkWithFastQueryApi()
{
  const string Query = "fast:/sitecore/content/*[@@key = 'home']";

  using (Sitecore.FakeDb.Db db = new Sitecore.FakeDb.Db 
    {
      new Sitecore.FakeDb.DbItem("home")
    })
  {
    Sitecore.Data.Items.Item homeItem = db.Database.SelectSingleItem(Query);

    Xunit.Assert.Equal(homeItem.Key, "home");
  }
}

Important:

Under the hood Sitecore Query is used. The Fast Query limitations are not applied to the result.