foo.cs

[Fact]
public void SomeOtherReallyBadTest()
{
    var context = new ConfigurableContext<FooContext>(ctx =>
    {
        ctx.Setup(x => x.Users, new List<User>());
    });

    var subject = new Foo(configurableContext); //Note the implicit conversion
    
    var usr = subject.SaveUser(new User { Name = "Joe" });
    
    context.WasSaved<User>(u => u.Name == "Joe");
}  

[Fact]
public void SomeReallyBadTest()
{
    var context = new ConfigurableContext<FooContext>(ctx =>
    {
        ctx.Setup(x => x.Users, new List<User>
        {
            new User
            {
                UserId = 1014,
                Username = "Joe",
                Password = "SecurePassword1ThatIsHashed!"
            }
        });
    });

    var subject = new Foo(configurableContext); //Note the implicit conversion
    
    var usr = subject.FindUser("Joe");
    usr.Should().NotBeNull();
}