updated xunit artifact handler to process bazel files
updated xunit artifact handler to process bazel files

Summary: This adds the patterns '*.bazel.xml' and '*.bazel.xml.dup.*' to the xunit artifact handler

Test Plan: unit tests

Reviewers: anupc

Reviewed By: anupc

Subscribers: changesbot, kylec

Differential Revision: https://tails.corp.dropbox.com/D223005

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