Add test execution time to `<test>` elements under `projects/tests` in the XML reports index file

Currently we have

```xml
<phpunit xmlns="https://schema.phpunit.de/coverage/1.0">
  <project source="%s">
    <tests>
      <test name="..." size="..." status="..."/>
```

This should be changed to

```xml
<phpunit xmlns="https://schema.phpunit.de/coverage/1.0">
  <project source="%s">
    <tests>
      <test name="..." size="..." status="..." time="..."/>
```

CC @theofidry