Injecting configuration into mock config object
Your mock of the Config object needs to also have a ::get method and return the appropriate value using your array. The core UnitTestCase class provides ::getConfigFactoryStub(array $configs), which does all of this automatically.
In your setup, create the config factory like this:
$this-&gt;configFactory = $this-&gt;getConfigFactoryStub([
  'mymodule.job' =&gt; ['bucketName' =&gt; 'test_bucket'],
]);

Unfortunately, the mocked config does not quite work like a real config object; namely, it doesn't accept dotted keys but only top-level keys. The following will not work:
$this-&gt;configFactory = $this-&gt;getConfigFactoryStub([
  'mymodule.settings' =&gt; [
    'key' =&gt; [
      'subkey' =&gt; [
        'subsubkey' =&gt; 'value',
      ],
    ],
  ],
]);
$this-&gt;container = new ContainerBuilder();
$this-&gt;container-&gt;set('config.factory', $this-&gt;configFactory);
\Drupal::setContainer($this-&gt;container);

\Drupal::config('mymodule.settings')-&gt;get('key.subkey.subsubkey');

This bug report shows a workaround: https://www.drupal.org/node/2862248
Update: The above problem was fixed in Drupal 8.5.0, so the workaround is no longer needed.