Total duration for flaky tests

Since flaky tests are often retried multiple times, this adds up to lots of extra time running tests.

To show this, the Flaky Tests page has a new column called Total Duration. This column shows the total execution time spent running a test in a given week:

This value is useful for prioritizing which flaky tests to fix first.

For example, you might have a test that's very flaky (i.e. it has a high flakiness score) but either doesn't take very long to run each time, or it doesn't run very often, or both. In comparison, you might have a test that is less flaky but takes a very long time to run -- so you'll probably want to fix that first.