Generated test.xml doesn't include the timestamp attribute

### Description of the bug:

The generated `test.xml` file doesn't include the `timestamp` attribute. Which is needed by our result backend. I wonder how hard is to fix this issue.

If I understand correctly the test.xml file is produced in [here](https://github.com/bazelbuild/bazel/blob/76e56d4b58f6985be4be725f291e35fa4fcb42fe/tools/test/generate-xml.sh). I don't see a mention to the timestamp in the source code.

### Which category does this issue belong to?

Core

### What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

_No response_

### Which operating system are you running Bazel on?

_No response_

### What is the output of `bazel info release`?

_No response_

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse HEAD` ?

```text

```

### If this is a regression, please try to identify the Bazel commit where the bug was introduced with bazelisk --bisect.

_No response_

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

_No response_

Start with tools/test/generate-xml.sh, which the issue identifies as the source of test.xml. Trace how the XML attributes are assembled and determine what timestamp value the result backend requires. Done means generated test.xml includes the required timestamp attribute; add or run a focused test if the surrounding test-generation workflow provides one.

Written by the indexing model from the issue text.