Adding additional test case for upload file button

After some recent breakage in the upload file button, I noticed that I
failed to test the breaking context (nonempty repo).

This MR backfills that context.

part of:
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/52755
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/53623
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/55548

spec/features/projects/show/user_uploads_files_spec.rb

@@ -34,33 +34,23 @@ RSpec.describe 'Projects > Show > User uploads files' do
     include_examples 'it uploads and commit a new file to a forked project'
   end
 
-  context 'with an empty repo' do
-    let(:project) { create(:project, :empty_repo, creator: user) }
-
-    context 'when in the empty_repo_upload experiment' do
-      before do
-        stub_experiments(empty_repo_upload: :candidate)
-
-        visit(project_path(project))
-      end
-
-      it 'uploads and commits a new text file', :js do
-        click_link('Upload file')
+  context 'when in the empty_repo_upload experiment' do
+    before do
+      stub_experiments(empty_repo_upload: :candidate)
 
-        drop_in_dropzone(File.join(Rails.root, 'spec', 'fixtures', 'doc_sample.txt'))
+      visit(project_path(project))
+    end
 
-        page.within('#modal-upload-blob') do
-          fill_in(:commit_message, with: 'New commit message')
-        end
+    context 'with an empty repo' do
+      let(:project) { create(:project, :empty_repo, creator: user) }
 
-        click_button('Upload file')
+      include_examples 'uploads and commits a new text file via "upload file" button'
+    end
 
-        wait_for_requests
+    context 'with a nonempty repo' do
+      let(:project) { create(:project, :repository, creator: user) }
 
-        expect(page).to have_content('New commit message')
-        expect(page).to have_content('Lorem ipsum dolor sit amet')
-        expect(page).to have_content('Sed ut perspiciatis unde omnis')
-      end
+      include_examples 'uploads and commits a new text file via "upload file" button'
     end
   end
 end

spec/support/shared_examples/features/project_upload_files_shared_examples.rb

@@ -94,3 +94,23 @@ RSpec.shared_examples 'it uploads and commit a new file to a forked project' do
     expect(page).to have_content('Sed ut perspiciatis unde omnis')
   end
 end
+
+RSpec.shared_examples 'uploads and commits a new text file via "upload file" button' do
+  it 'uploads and commits a new text file via "upload file" button', :js do
+    find('.js-upload-file-experiment-trigger', text: 'Upload file').click
+
+    drop_in_dropzone(File.join(Rails.root, 'spec', 'fixtures', 'doc_sample.txt'))
+
+    page.within('#modal-upload-blob') do
+      fill_in(:commit_message, with: 'New commit message')
+    end
+
+    click_button('Upload file')
+
+    wait_for_requests
+
+    expect(page).to have_content('New commit message')
+    expect(page).to have_content('Lorem ipsum dolor sit amet')
+    expect(page).to have_content('Sed ut perspiciatis unde omnis')
+  end
+end