Test that set will propagate.
public function testSet() {
$this->chain
->set('test', 123);
$cached = $this->firstBackend
->get('test');
$this
->assertNotSame(FALSE, $cached, 'Test key is in the first backend');
$this
->assertSame(123, $cached->data, 'Test key has the right value');
$cached = $this->secondBackend
->get('test');
$this
->assertNotSame(FALSE, $cached, 'Test key is in the second backend');
$this
->assertSame(123, $cached->data, 'Test key has the right value');
$cached = $this->thirdBackend
->get('test');
$this
->assertNotSame(FALSE, $cached, 'Test key is in the third backend');
$this
->assertSame(123, $cached->data, 'Test key has the right value');
}