Refactor search_tab? -> show_user_search_tab?

This method was only ever used with the argument :members but further it
seems to have been half-implemented generically and the argument wasn't
even being used so it was just causing confusion. Since this is only
meant to be checking if the users tab should show it is now a bit more
explicit.

app/helpers/search_helper.rb

@@ -273,7 +273,7 @@ module SearchHelper
     sanitize(html, tags: %w(a p ol ul li pre code))
   end
 
-  def search_tabs?(tab)
+  def show_user_search_tab?
     return false if Feature.disabled?(:users_search, default_enabled: true)
 
     if @project

app/views/search/_category.html.haml

 - users = capture_haml do
-  - if search_tabs?(:members)
+  - if show_user_search_tab?
     = search_filter_link 'users', _("Users")
 
 .scrolling-tabs-container.inner-page-scroll-tabs.is-smaller

spec/helpers/search_helper_spec.rb

@@ -271,4 +271,50 @@ describe SearchHelper do
       expect(link).to have_css('li[data-foo="bar"]')
     end
   end
+
+  describe '#show_user_search_tab?' do
+    subject { show_user_search_tab? }
+
+    context 'when users_search feature is disabled' do
+      before do
+        stub_feature_flags(users_search: false)
+      end
+
+      it { is_expected.to eq(false) }
+    end
+
+    context 'when project search' do
+      before do
+        @project = :some_project
+
+        expect(self).to receive(:project_search_tabs?)
+          .with(:members)
+          .and_return(:value)
+      end
+
+      it 'delegates to project_search_tabs?' do
+        expect(subject).to eq(:value)
+      end
+    end
+
+    context 'when not project search' do
+      context 'when current_user can read_users_list' do
+        before do
+          allow(self).to receive(:current_user).and_return(:the_current_user)
+          allow(self).to receive(:can?).with(:the_current_user, :read_users_list).and_return(true)
+        end
+
+        it { is_expected.to eq(true) }
+      end
+
+      context 'when current_user cannot read_users_list' do
+        before do
+          allow(self).to receive(:current_user).and_return(:the_current_user)
+          allow(self).to receive(:can?).with(:the_current_user, :read_users_list).and_return(false)
+        end
+
+        it { is_expected.to eq(false) }
+      end
+    end
+  end
 end