Populate timestamp in test.xml (#4259)

**What type of PR is this?**
Feature

**What does this PR do? Why is it needed?**
The `<testsuite>` element in JUnit XML has a `timestamp` attribute, but
the `<testcase>` doesn't, while Go test's json has the timestamp for
every event. This PR find the smallest timestamp in the json events and
use that as the `timestamp` of `<testsuite>`.

**Test plan**
* Unit test
* Verify that the timestamps are different with `--runs_per_test=10`
2 files changed