Output information about skipped tests in xml output

[mvoorsluys]

This pull request adds two things to the xml output:

• The number of skipped tests inside a suite. (As a property, like the number of failures)
• On skipped tests it also adds a skipped element to the xml.

I followed the JUnit description in https://llg.cubic.org/docs/junit/

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[zhangxy988]

Please sign the CLA and make sure the tests pass.

[mvoorsluys]

@googlebot I signed it!

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[mvoorsluys]

@zhangxy988 The tests are passing, but the travis check remains pending. I don't know how I can solve this. The CLA has been resolved.

[mvoorsluys]

Fixes #2728

[CJ-Johnson]

Thank you, we have started internal review. Please don’t push any more changes into this PR as they might be overwritten. (313176466)