The key piece in the unit tests is re-querying the KAV record prior to creating a standard controller based on it. This is because that field is set by the system when the KAV is inserted and will be NULL and cause an expcetion if not queried for.

Create your test Case and the rest of the code should work in conjunction with the controller Francois provided previously.

static testMethod void conArticles_Coverage() {  

     // Create new Case

     // Create new Article
     Solution__kav solKAV = new Solution__kav(Title='Test Article',URLName='Test-Article',PublishStatus='Online');
     insert solKAV;

     // Re-query the record to get the parent KnowledgeArticle ID for the new KAV.
     solKAV = [SELECT id, KnowledgeArticleId FROM Solution__kav WHERE id=: solKav.Id limit 1];

     // Create page reference and add our values as parameters for the controller
     // Case c was created previously
     ApexPages.StandardController sc = new ApexPages.StandardController(solKAV);    	
     PageReference pageRef = Page.showArticle_Solution;
     pageRef.getParameters().put('caseid',c.Id);
     pageRef.getParameters().put('id',solKav.KnowledgeArticleId);

     Test.setCurrentPage(pageRef);

     // Create an instance on our controller and assert that our IDs were set properly
     conArticles con = new conArticles(sc);
     system.assertEquals(con.caseId,c.Id);
     system.assertEquals(con.kaId,solKAV.KnowledgeArticleId);

     // Call the attachToCase method and assert that our Case now has a new CaseArticle
     con.attachToCase();
     system.assert([SELECT count() FROM CaseArticle WHERE CaseId =: c.Id] > 0);
}