[Telemetry] Move TestStreamOutput to unittest_util.

[Telemetry] Move TestStreamOutput to unittest_util.

Motivation: when I make benchmark_runner._PrintBenchmarkList a public method,
I want to provide unittest coverage for the method. I would want to reuse the
logic of TestStreamOutput for writing that unittest.

BUG=482090
Committed: https://crrev.com/3b678eae31007c956a67f507172be4f287725c95
Cr-Commit-Position: refs/heads/master@{#327610}

[nednguyen, 2015-04-29 16:49:49]

nednguyen@google.com changed reviewers:
+ dtu@chromium.org, eakuefner@chromium.org

[eakuefner, 2015-04-29 23:10:42]

lgtm

[nednguyen, 2015-04-29 23:14:05]

The CQ bit was checked by nednguyen@google.com

[commit-bot: I haz the power, 2015-04-29 23:14:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1112773003/1

[commit-bot: I haz the power, 2015-04-29 23:20:44]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-29 23:21:37]

Patchset 1 (id:??) landed as
https://crrev.com/3b678eae31007c956a67f507172be4f287725c95
Cr-Commit-Position: refs/heads/master@{#327610}