cygprofile: switch run_tests to vpython and revert psutil import workaround

This switches tools/cygprofile/run_tests to vpython, which should have
been done as part of http://crrev.com/546845.

After the vpython switch the workaround in http://crrev.com/546845
should not be necessary, so revert this commit as well:
> cygprofile: Fix unittests for cygprofile on CQ.
>
> orderfile_generator_backend_unittests imports
> orderfile_generator_backend, which transitively requires psutil, which
> is not available on the CQ bots.
>
> As psutil is not used in unit tests, handle the import error to
> allow unit tests to run on CQ, and CLs in //tools/cygprofile to pass
> presubmit checks.
>
> Change-Id: Id06bd2b7936509ef44a02d7b6497fa818d439291
> Reviewed-on: https://chromium-review.googlesource.com/985975
> Commit-Queue: Benoit L <lizeb@chromium.org>
> Reviewed-by: Egor Pasko <pasko@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#546790}

Bug: None
Change-Id: Ib0737acf652b283c212a61a2bae83a9c373c9fbf
Reviewed-on: https://chromium-review.googlesource.com/988072
Reviewed-by: Benoit L <lizeb@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547659}

• tools/cygprofile/orderfile_generator_backend.py[diff]
• tools/cygprofile/run_tests[diff]