[ci] improvements to test stats uploading
Two improvements that are useful for `testsuite` uploading:
- When there are multiple tags of the same name, coalesce them into a
list. This allows us to capture, e.g. when a `testsuite` contains
multiple `testcase`s.
- Be able to skip a tag. We don't really want the inner `testcase` tags,
since we upload those separately.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79364
Approved by: https://github.com/janeyx99
