tools: Fix running full-stack tests from tools/ directory.

Previously these tests required you to run them with the root of the
Zulip repository as the current working directory, just due to
sloppiness.

We clean this up, while also making the path handling more consistent
and involving less fragile code.

Fixes #4169.

tools/test-help-documentation.py

@@ -8,6 +8,7 @@ import subprocess
 
 import time
 
+ZULIP_PATH = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
 
 # check for the venv
 from lib import sanity_check
@@ -21,9 +22,8 @@ parser.add_option('--force', default=False,
                   help='Run tests despite possible problems.')
 (options, args) = parser.parse_args()
 
-TOOLS_DIR = os.path.dirname(os.path.abspath(__file__))
-sys.path.insert(0, os.path.dirname(TOOLS_DIR))
-
+os.chdir(ZULIP_PATH)
+sys.path.insert(0, ZULIP_PATH)
 from tools.lib.test_server import test_server_running
 
 subprocess.check_call(['mkdir', '-p', 'var/help-documentation'])