Fix test_id issue in :button selector and add test

lib/capybara/selector/definition/button.rb

@@ -57,7 +57,7 @@ Capybara.add_selector(:button, locator_type: [String, Symbol]) do
       XPath.attr(:title).is(locator),
       (XPath.attr(:id) == XPath.anywhere(:label)[XPath.string.n.is(locator)].attr(:for)),
       (XPath.attr(:'aria-label').is(locator) if config.enable_aria_label),
-      (XPath.attr(test_id) == locator if config.test_id)
+      (XPath.attr(config.test_id) == locator if config.test_id)
     ].compact.inject(&:|)
   end
 end

lib/capybara/spec/session/click_link_or_button_spec.rb

@@ -38,6 +38,15 @@ Capybara::SpecHelper.spec '#click_link_or_button' do
     expect(extract_results(@session)['first_name']).to eq('John')
   end
 
+  context 'with test_id' do
+    it 'should click on a button' do
+      Capybara.test_id = 'data-test-id'
+      @session.visit('/form')
+      @session.click_link_or_button('test_id_button')
+      expect(extract_results(@session)['first_name']).to eq('John')
+    end
+  end
+
   context 'with :exact option' do
     context 'when `false`' do
       it 'clicks on approximately matching link' do

lib/capybara/spec/views/form.erb

@@ -449,7 +449,7 @@ New line after and before textarea tag
 
   <p>
     <input type="button" aria-label="Aria button" name="form[fresh]" id="fresh_btn" value="i am fresh"/>
-    <input type="submit" name="form[awesome]" id="awe123" title="What an Awesome Button" value="awesome"/>
+    <input type="submit" name="form[awesome]" id="awe123" data-test-id="test_id_button" title="What an Awesome Button" value="awesome"/>
     <input type="submit" name="form[crappy]" id="crap321" value="crappy"/>
     <input type="image" name="form[okay]" id="okay556" title="Okay 556 Image" value="okay" alt="oh hai thar"/>
     <button type="submit" id="click_me_123" title="Click Title button" value="click_me">Click me!</button>