Sort code coverage graph by dates

What does this MR do?

Follow up to #33743 (closed) because we discovered that the graph was rendering in reverse order. The API passed the data in the order on purpose, so we adjust them in the FE.

It's a fairly straightforward change: we add a computed property that sorts the array. We could have opted for using something like reverse since we know it's in descending order, but it feels more robust to do a full on date comparison.

I also added a simple test that check that the mock data, which is now in descending order, gets reverse to the correct order.

app/assets/javascripts/pages/projects/graphs/components/code_coverage.vue

@@ -70,8 +70,6 @@ export default {
         return [];
       }
 
-      // The data is returned descending order of dates for the CSV.
-      // so we sort them by ascending order fo the graph
       return [...this.selectedDailyCoverage.data].sort(
         (a, b) => new Date(a.date) - new Date(b.date),
       );