Test that delete will propagate.
public function testDelete() {
$this->chain
->set('test', 5);
$cached = $this->firstBackend
->get('test');
$this
->assertNotSame(FALSE, $cached, 'Test key has been added to the first backend');
$cached = $this->secondBackend
->get('test');
$this
->assertNotSame(FALSE, $cached, 'Test key has been added to the first backend');
$cached = $this->thirdBackend
->get('test');
$this
->assertNotSame(FALSE, $cached, 'Test key has been added to the first backend');
$this->chain
->delete('test');
$cached = $this->firstBackend
->get('test');
$this
->assertSame(FALSE, $cached, 'Test key is removed from the first backend');
$cached = $this->secondBackend
->get('test');
$this
->assertSame(FALSE, $cached, 'Test key is removed from the second backend');
$cached = $this->thirdBackend
->get('test');
$this
->assertSame(FALSE, $cached, 'Test key is removed from the third backend');
}