[e2e] Adds support for setting a timeout
[e2e] Adds support for setting a timeout

This CL allows the setting of a TIMEOUT env variable to control test
timeout values.

Includes a drive-by fix on only outputting actual and expected values
when they are set.

Bug: 1062614
Change-Id: Idf444778a8a7292052a489a758b57bd2cfac81f9
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2123470
Reviewed-by: Tim van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>
Commit-Queue: Paul Lewis <8915f5d5c39a08c5a3e8a8d7314756e4580816f9@chromium.org>

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