tools: Create more consistent checks for venv.

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.

tools/test-run-dev

@@ -10,6 +10,9 @@ import signal
 import subprocess
 
 from six.moves import range
+# check for the venv
+from lib import sanity_check
+sanity_check.check_venv(__file__)
 from typing import IO