How to unit test using a ViewModelLocator

I've created a custom view model locator using Autofac and set it up normally through the App.xaml like most of them are usually used. My problem is how do I unit test now? I'm getting an error every time I try to test a method that initializes a view

In my app.xaml:

<desktop:ViewModelLocator xmlns:local="clr-namespace:MyProject.Desktop" x:Key="ViewModelLocator" />

In each view:

DataContext="{Binding MyFirstViewModel, Source={StaticResource ViewModelLocator}}"

The Unit Test Error:

{"Cannot find resource named 'ViewModelLocator'. Resource names are case sensitive."}

I understand why cause when you unit test, there really isn't an instance of the actual App so what is a good way around this problem?

ViewModelLocator Code:

/// <summary>
/// Autofac object container
/// </summary>
private readonly IContainer objectContainer;

#region Constructor

/// <summary>
/// Constructor for view model locator
/// </summary>
public ViewModelLocator()
{
    objectContainer = App.ObjectContainer;
    //objectContainer.BeginLifetimeScope();
}

#endregion

#region Properties

/// <summary>
/// Gets the resolved instance of a main window view model
/// </summary>
public MainWindowViewModel MainWindowViewModel
{
    get
    {
        return objectContainer.Resolve<MainWindowViewModel>();
    }
}

public FirstViewModel MyFirstViewModel 
{
    get
    {
        return objectContainer.Resolve<FirstViewModel>();
    }
}

public SecondViewModel MySecondViewModel 
{
    get
    {
        return objectContainer.Resolve<SecondViewModel>();
    }
}