Test that the delete all operation is propagated to all backends in the chain.
public function testDeleteAllPropagation() {
// Set both expiring and permanent keys.
$this->chain
->set('test1', 1, CacheBackendInterface::CACHE_PERMANENT);
$this->chain
->set('test2', 3, time() + 1000);
$this->chain
->deleteAll();
$this
->assertTrue($this->firstBackend
->isEmpty(), 'First backend is empty after delete all.');
$this
->assertTrue($this->secondBackend
->isEmpty(), 'Second backend is empty after delete all.');
$this
->assertTrue($this->thirdBackend
->isEmpty(), 'Third backend is empty after delete all.');
}