EZP-21802: eZImageManagerTest suite only gets executed when ImageMagick and GD are present

The `eZImageManagerTest` Suite only gets executed when ImageMagick *and* GD are installed, not when only one of them is present, due to this check:

```php
if ( !self::gdIsEnabled() or !self::imageMagickIsEnabled() )
            $this->markTestSkipped( 'GD and/or ImageMagick are not enabled' );
```

This PR aims to fix that.

Cheers
:octocat: Jérôme