Fix flaky spec

Running locally this spec failed me quite a few times.

I think it may be because this screen loads a graphic and it seems to take
a while to load (<%= render "admin/stats/graph" ...  %>)

I have run several tests in a row with success when removing this graph
from the view, or when adding a sleep.

It seems that adding the have_css of the chart id also seems to get me
through the spec.

spec/system/admin/stats_spec.rb

@@ -106,6 +106,7 @@
           click_link "Supporting phase"
         end
 
+        expect(page).to have_css "#graph"
         expect(page).to have_content "VOTES\n3"
         expect(page).to have_content "PARTICIPANTS\n2"
         expect(page).to have_link "Go back", count: 1