Make the completion events test fail less

test/completion/test_completion_events.vader

@@ -32,8 +32,16 @@ Before:
     endfunction
 
     let g:ale_completion_delay = 0
-    call ale#completion#Queue()
-    sleep 1m
+
+    " Run this check a few times, as it can fail randomly.
+    for g:i in range(has('nvim-0.3') || has('win32') ? 5 : 1)
+      call ale#completion#Queue()
+      sleep 1m
+
+      if g:get_completions_called is a:expect_success
+        break
+      endif
+    endfor
 
     AssertEqual a:expect_success, g:get_completions_called
   endfunction