Replace assertions involving calls to isset() with assertArrayHasKey()/assertArrayNotHasKey

Problem/Motivation

As title

Proposed resolution

Remove redundant assertion messages at the same time.

Example:

-  $this->assertTrue(isset($behaviors['access_denied']['label']), 'The access denied plugin has a label');
+ $this->assertArrayHasKey('label', $behaviors['access_denied'], 'The access denied plugin has a label');