Test class for contentdocumentlink

Rather than querying ContentDocumentId from ContentVersion, query that from ContentDocument and use that as contentlink.ContentDocumentId=documents[0].Id;

List<ContentDocument> documents = [
    SELECT Id, Title, LatestPublishedVersionId 
    FROM ContentDocument
];

ContentDocumentLink contentlink=new ContentDocumentLink();
contentlink.LinkedEntityId=availableRateDeck.id;
contentlink.ShareType= 'C';
contentlink.LinkedEntityId = availableRateDeck.Id; 
contentlink.ContentDocumentId=documents[0].Id;
contentlink.Visibility = 'AllUsers'; 
insert contentlink;

Just to let you know, following piece of code works perfectly for me

Account acct = new Account(Name='TEST_ACCT');
insert acct;

ContentVersion contentVersion = new ContentVersion(
  Title = 'Penguins',
  PathOnClient = 'Penguins.jpg',
  VersionData = Blob.valueOf('Test Content'),
  IsMajorVersion = true
);
insert contentVersion;    
List<ContentDocument> documents = [
    SELECT Id, Title, LatestPublishedVersionId 
    FROM ContentDocument
];

//create ContentDocumentLink  record 
ContentDocumentLink cdl = New ContentDocumentLink();
cdl.LinkedEntityId = acct.id;
cdl.ContentDocumentId = documents[0].Id;
cdl.shareType = 'V';
insert cdl;