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This should avoid import problems from various catapult dependencies
that are pulled via src/.vpython. One such problem was recently
mentioned in http://crrev.com/546790.

Also for consistency:
1. make all unittests executable
2. remove shebang and exec bit for the scripts that never produce output
   (utilities)

To read more about vpython:
  https://chromium.googlesource.com/infra/infra/+/master/doc/users/vpython_one_page.md

Change-Id: I167a13f03b1f4a0c95d57557055840fa35db7f41
Reviewed-on: https://chromium-review.googlesource.com/986264Reviewed-by: Benoit L <lizeb@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546845}