Ensure get values propagation to previous backends.
public function testGetHasPropagated() {
$this->chain
->get('t23');
$cached = $this->firstBackend
->get('t23');
$this
->assertNotIdentical(FALSE, $cached, 'Test 2 has been propagated to the first backend');
$this->chain
->get('t3');
$cached = $this->firstBackend
->get('t3');
$this
->assertNotIdentical(FALSE, $cached, 'Test 3 has been propagated to the first backend');
$cached = $this->secondBackend
->get('t3');
$this
->assertNotIdentical(FALSE, $cached, 'Test 3 has been propagated to the second backend');
}