I know why! run_slave.py assumes dependencies in the system python! whee!

Nope! It's because tests/slave_test.py OVERRIDES SITE.PY. This is like 100% don't-do-it territory.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/build/+/317b9f72d3790d37520ca7d19f5613c739bb0dd8

commit 317b9f72d3790d37520ca7d19f5613c739bb0dd8
Author: Robert Iannucci <iannucci@chromium.org>
Date: Tue Jan 09 23:28:12 2018

[vpython] Fix tests/slave_test.py to work under VirtualEnv.

Instead of overriding site.py (which is used by VirtualEnv to configure
all the VirtualEnv magic), this writes a boostrap script which does a
bit of logic before exec'ing into the the run_slave.py file.

R=dnj@chromium.org, tandrii@chromium.org

Bug:  800485 
Change-Id: Id7523c8e4b33e96a7585bc53cbcce8f47abc3442
Reviewed-on: https://chromium-review.googlesource.com/858233
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>

[modify] https://crrev.com/317b9f72d3790d37520ca7d19f5613c739bb0dd8/tests/slave_test.py