test-js-with-node: Show correct coverage link in color.

We figure out the dev host using the same logic as
dev_settings.py, so that we don't use wrong things
like 127.0.0.1 for droplet users.

And we display the link in cyan.

tools/test-js-with-node

@@ -2,6 +2,7 @@
 import argparse
 import glob
 import os
+import pwd
 import subprocess
 import sys
 from typing import Dict, Any
@@ -17,7 +18,7 @@ sanity_check.check_venv(__file__)
 # Import this after we do the sanity_check so it doesn't crash.
 import ujson
 
-from zulint.printer import GREEN, BOLDRED, ENDC
+from zulint.printer import CYAN, GREEN, BOLDRED, ENDC
 
 INDEX_JS = 'frontend_tests/zjsunit/index.js'
 NODE_COVERAGE_PATH = 'var/node-coverage/coverage-final.json'
@@ -174,6 +175,26 @@ from tools.lib.test_script import assert_provisioning_status_ok
 
 assert_provisioning_status_ok(options.force)
 
+def get_dev_host() -> str:
+    # See similar code in dev_settings.py.  We only use
+    # this to report where you can find coverage reports.
+    # We duplicate the code here to avoid depending on
+    # Django.
+
+    host = os.getenv('EXTERNAL_HOST')
+    if host is not None:
+        return host
+
+    user_id = os.getuid()
+    user_name = pwd.getpwuid(user_id).pw_name
+    if user_name == "zulipdev":
+        # For our droplets, we use the external hostname by default.
+        return os.uname()[1].lower() + ":9991"
+    else:
+        # For local development environments, we use localhost by
+        # default, via the "zulipdev.com" hostname.
+        return 'zulipdev.com:9991'
+
 def print_error(msg: str) -> None:
     print(BOLDRED + 'ERROR:' + ENDC + ' ' + msg)
 
@@ -308,7 +329,9 @@ if options.coverage and ret == 0:
 print()
 if ret == 0:
     if options.coverage:
-        print("View coverage reports at http://127.0.0.1:9991/node-coverage/index.html")
+        reports_location = 'http://{}/node-coverage/index.html'.format(get_dev_host())
+        print('View coverage reports at ' + CYAN + reports_location + ENDC)
+
     print(GREEN + "Test(s) passed. SUCCESS!" + ENDC)
 else:
     print(BOLDRED + "FAIL - Test(s) failed" + ENDC)