This (1) changes test_server to use the common `check_venv` method and
(2) improves check_venv to provide a clearer error message in the case
that you're inside Vagrant but not in a venv.
Tweaked by tabbott to borrow logic from run_dev.py.
This helps make the Zulip development environment somewhat more robust
to new contributors, since it will give them a nice warning if they
try running any of our development tools outside the Zulip virtualenv.
Fixes#3468.