[9.x] Add empty directory assertion to Storage::fake()

[JayBizzle]

I was trying to assert that a given directory was empty in some tests and was surprised to see that there was no bulit-in assertion to handle this.

This PR proposes adding an assertDirectoryEmpty($path) method to the Storage fake.

Obviously this is just a cleaner version of doing...

$this->assertEmpty(Storage::disk('temp')->allFiles('/foo'));

vs

Storage::disk('temp')->assertDirectoryEmpty('/foo');

[taylorotwell]

What is the behavior if there are no files within the directory but there are other sub-directories?

[JayBizzle]

What is the behavior if there are no files within the directory but there are other sub-directories?

The assertion will fail, correctly IMO as the directory is not empty if it contains other folders

Storage::fake('temp');

Storage::disk('temp')->put('/foo/bar.txt', 'string');

Storage::disk('temp')->assertDirectoryEmpty('/'); // fail