InsertContentDocumentLinkTest.cls

@IsTest(IsParallel =false)  // ContentWorkspace
public class InsertContentDocumentLinkTest {
    @IsTest
    static void do_a_thing() {
        CollaborationGroup unlistedGroup = new CollaborationGroup(
            Name = 'Unlisted',
            CollaborationType = 'Unlisted'
        );
        insert unlistedGroup;

        ContentVersion docVer = new ContentVersion(
            Title ='My Doc', 
            PathOnClient = 'test.csv', 
            VersionData = EncodingUtil.base64Decode('Content Version lib test')
        );
        insert docVer;

        ContentDocument doc = [SELECT Id FROM ContentDocument];
        
        ContentDocumentLink commlink = new ContentDocumentLink(
            ContentDocumentId = doc.Id, 
            LinkedEntityId = unlistedGroup.Id, 
            ShareType = 'V', 
            Visibility = 'AllUsers'
        );
        insert commlink;

        System.assertNotEquals(null, commLink.Id);
    }
}