test_server: Improve warning in venv check.

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.

tools/lib/sanity_check.py

@@ -1,6 +1,8 @@
 #!/usr/bin/env python
 from __future__ import print_function
 
+import os
+import pwd
 import sys
 
 def check_venv(filename):
@@ -9,5 +11,10 @@ def check_venv(filename):
         import ujson
     except ImportError:
         print("You need to run %s inside a Zulip dev environment." % (filename,))
+        user_id = os.getuid()
+        user_name = pwd.getpwuid(user_id).pw_name
+        if user_name != 'vagrant' and user_name != 'zulipdev':
             print("If you are using Vagrant, you can `vagrant ssh` to enter the Vagrant guest.")
+        else:
+            print("You can `source /srv/zulip-venv/bin/activate` to enter the Zulip development environment.")
         sys.exit(1)

tools/lib/test_server.py

@@ -10,14 +10,12 @@ from contextlib import contextmanager
 if False:
     from typing import (Any, Iterator)
 
-try:
-    import django
-    import requests
-except ImportError as e:
-    print("ImportError: {}".format(e))
-    print("You need to run the Zulip tests inside a Zulip dev environment.")
-    print("If you are using Vagrant, you can `vagrant ssh` to enter the Vagrant guest.")
-    sys.exit(1)
+# Verify the Zulip venv is available.
+from tools.lib import sanity_check
+sanity_check.check_venv(__file__)
+
+import django
+import requests
 
 TOOLS_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
 if TOOLS_DIR not in sys.path: