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Test runner will copy attachments from xcresult to output folder
(isolated output in infra) for all failed tests in each attempt.
Screenshots will be collected only from failing steps to avoid noise.
Other attachments will be collected from every step.
- Put the copy logic in private methods and called these in public
collect_test_results.
- Improved and unified variable naming where touched.
- Removed unused plist check where touched.
- Fixed and added tests for xcode_log_parser.

Bug: 1047704
Change-Id: I448c785a256702d93698e5c73ef89b7477efbc87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2473499Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Zhaoyang Li <zhaoyangli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819543}