Adds an npmignore file for the tests

[orta]

Before:

~/d/p/o/j/domhandler  $ npm pack                                                                                                                       2 changed files  v2.4.2 
npm notice 
npm notice 📦  domhandler@2.4.2
npm notice === Tarball Contents === 
npm notice 817B  package.json                                
npm notice 5B    .npmignor                                   
npm notice 152B  .travis.yml                                 
npm notice 4.1kB domhandler-2.4.2.tgz                        
npm notice 5.2kB index.js                                    
npm notice 1.3kB LICENSE                                     
npm notice 2.7kB readme.md                                   
npm notice 443B  lib/element.js                              
npm notice 915B  lib/node.js                                 
npm notice 1.2kB test/cases/01-basic.json                    
npm notice 296B  test/cases/02-single_tag_1.json             
npm notice 295B  test/cases/03-single_tag_2.json             
npm notice 715B  test/cases/04-unescaped_in_script.json      
npm notice 363B  test/cases/05-tags_in_comment.json          
npm notice 323B  test/cases/06-comment_in_script.json        
npm notice 407B  test/cases/07-unescaped_in_style.json       
npm notice 341B  test/cases/08-extra_spaces_in_tag.json      
npm notice 324B  test/cases/09-unquoted_attrib.json          
npm notice 246B  test/cases/10-singular_attribute.json       
npm notice 686B  test/cases/11-text_outside_tags.json        
npm notice 160B  test/cases/12-text_only.json                
npm notice 312B  test/cases/13-comment_in_text.json          
npm notice 368B  test/cases/14-comment_in_text_in_script.json
npm notice 377B  test/cases/15-non-verbose.json              
npm notice 774B  test/cases/16-normalize_whitespace.json     
npm notice 279B  test/cases/17-xml_namespace.json            
npm notice 234B  test/cases/18-enforce_empty_tags.json       
npm notice 311B  test/cases/19-ignore_empty_tags.json        
npm notice 377B  test/cases/20-template_script_tags.json     
npm notice 467B  test/cases/21-conditional_comments.json     
npm notice 810B  test/cases/22-lowercase_tags.json           
npm notice 3.4kB test/cases/23-dom-lvl1.json                 
npm notice 1.9kB test/cases/24-with-start-indices.json       
npm notice 1.9kB test/cases/25-with-end-indices.json         
npm notice 1.6kB test/tests.js                               
npm notice === Tarball Details === 
npm notice name:          domhandler                              
npm notice version:       2.4.2                                   
npm notice filename:      domhandler-2.4.2.tgz                    
npm notice package size:  12.6 kB                                 
npm notice unpacked size: 34.1 kB                                 
npm notice shasum:        7ea9fe808c6b0b0ae9ff826203ee16311b183519
npm notice integrity:     sha512-LsSrRsISNgNg/[...]6s2QA1UuUw8UA==
npm notice total files:   35                                      
npm notice 
domhandler-2.4.2.tgz
~/d/p/o/j/domhandler  $                                                                                                                                2 changed files  v2.4.2 

After:

    pack
 ▲ ~/d/p/o/j/domhandler  $ npm pack                                                                                                                     1 changed file  v2.4.2 
npm notice 
npm notice 📦  domhandler@2.4.2
npm notice === Tarball Contents === 
npm notice 817B  package.json  
npm notice 152B  .travis.yml   
npm notice 5.2kB index.js      
npm notice 1.3kB LICENSE       
npm notice 2.7kB readme.md     
npm notice 443B  lib/element.js
npm notice 915B  lib/node.js   
npm notice === Tarball Details === 
npm notice name:          domhandler                              
npm notice version:       2.4.2                                   
npm notice filename:      domhandler-2.4.2.tgz                    
npm notice package size:  4.1 kB                                  
npm notice unpacked size: 11.5 kB                                 
npm notice shasum:        e290f8b77779e39e26b2b6e1bbd4e5c2923a82c8
npm notice integrity:     sha512-oF1eWQ0RxlRFi[...]xKOLFh/jQq8+A==
npm notice total files:   7