Try deleting a directory with some files.
function testDirectory() {
// A directory to operate on.
$directory = $this
->createDirectory();
$filepathA = $directory . '/A';
$filepathB = $directory . '/B';
file_put_contents($filepathA, '');
file_put_contents($filepathB, '');
// Delete the directory.
$this
->assertTrue(file_unmanaged_delete_recursive($directory), 'Function reported success.');
$this
->assertFalse(file_exists($filepathA), 'Test file A has been deleted.');
$this
->assertFalse(file_exists($filepathB), 'Test file B has been deleted.');
$this
->assertFalse(file_exists($directory), 'Directory has been deleted.');
}