this commit also adds the LastDuration field to the TestResult
GQL model. The reason we're using latest_run and not the updated_at
field of the object is because in the future we want to parse the
timestamp from the JUnit XML file so latest_run will be tied to that
and not the time at which we process the test results.

- TestResultsHeaders represents the information displayed in the Test
Analytics UI that is related to test results, total duration of tests,
accumulated duration of the 5% slowest tests, total number of failures,
and total number of skips

- a field of type TestResultsHeaders was added to the Repository model

- rename the TestResultsHeaders GQL model to TestResultsAggregates
- create the FlakeAggregates that contains flakeCount and flakeRate
  and uses the flaky_failure_count field from the DailyTestRollup

we want to be able to filter by flaky tests, failed tests and the
slowest tests, because we want the clickable headers in the UI

This adds PercentChange fields to both the TestResultsAggregates
and the FlakeAggregates. These work by getting the aggregates
for the previous historical period and comparing them with the
ones for the current historical period and getting a percentage
difference.