Test that the chain is not empty when at least one backend has data.
public function testNotEmptyIfOneBackendHasTheKey() {
$this
->assertFalse($this->chain
->isEmpty(), 'Chain is not empty');
// This is the only test that needs to start with an empty chain.
$this->chain
->deleteAll();
$this
->assert($this->chain
->isEmpty(), 'Chain have been emptied by the deleteAll() call');
$this->secondBackend
->set('test', 5);
$this
->assertFalse($this->chain
->isEmpty(), 'Chain is not empty anymore now that the second backend has something');
}