Parse timestamp attr from testcase in junit xml.

[n-rook]

timestamp is just read as a string, as is already the case for the timestamp field on testsuite elements. A lot of test runners don't set this attribute, but some do, such as Terraform:

https://developer.hashicorp.com/terraform/cli/commands/test

The goal here is to allow other software that uses the testgrid xml-parsing library to read this attribute. I do not anticipate this attribute being used by testgrid itself, since I do not think testgrid really operates on this small of a scale. Although of course it could be used someday.

[google-oss-prow]

Hi @n-rook. Thanks for your PR.

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[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chases2, michelle192837, n-rook

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