Laravel dependency injection into custom non-controller class fails in PHPUnit - laravel

Question

all.
For a Laravel project I'm working on, I've started to use Dependency Injection in order to mock classes in my tests. However, I've found that if I try to inject into a custom class with no explicit parent, two things are true:
The dependency injection works correctly when running the application
The injection fails when running tests in PHPUnit
Here is some sample code similar to what I'm using:
DemoController
// The controller we're testing
class DemoController extends Controller
{
// The injection and constructor
private $helpLayer1;
public function __construct(HelpLayer1 $helpLayer1)
{
$this->helpLayer1 = $helpLayer1;
}
...
// The test function I call
public function testDeps()
{
$this->helpLayer1->testLayer1();
}
}
HelperLayer1
// Our first helper class
class HelperLayer1
{
private $helpLayer2;
public function __construct(HelpLayer2 $helpLayer2)
{
$this->helpLayer2 = $helpLayer2;
}
...
// The testing function
public function testLayer1()
{
// When called via route, this dumps the actual object
// When called via test, this returns null
dd($this->helperLayer2);
}
}
Helper1ServiceProvider
class Helper1ServiceProvider extends ServiceProvider
{
public function register()
{
$this->app->bind('HelperLayer1', function()
{
return new HelperLayer1(App::make('HelperLayer2'));
});
}
[OR]
public function register()
{
$this->app->bind('HelperLayer1', 'HelperLayer1');
}
}
Helper2ServiceProvider
class Helper2ServiceProvider extends ServiceProvider
{
public function register()
{
$this->app->bind('HelperLayer2', 'HelperLayer2');
}
}
I'm relatively new to using DI, so I'm not entirely sure that this set-up is correct, but I'm at a loss.
Any help would be greatly appreciated! Thank you!