Add a test against infinite prompt during cluster setup

Prior to 1254a2f and when running
"dcos cluster setup" command as non-interactive, the process would run
and ask for user input forever.

This adds an integration test which fails if the process doesn't exit by
itself with an error within 15 seconds.

https://jira.mesosphere.com/browse/DCOS-15590

cli/tests/integrations/test_cluster.py

@@ -1,4 +1,5 @@
 import json
+import subprocess
 
 from .helpers.common import assert_command, exec_command
 
@@ -43,3 +44,24 @@ def test_rename():
 
     # rename back to original name
     assert_command(['dcos', 'cluster', 'rename', new_name, name])
+
+
+def test_setup_noninteractive():
+    """
+    Run "dcos cluster setup" command as non-interactive with a 30 sec timeout.
+    This makes sure the process doesn't prompt for input forever (DCOS-15590).
+    """
+
+    try:
+        returncode, stdout, _ = exec_command(
+            ['dcos',
+             'cluster',
+             'setup',
+             'https://dcos.snakeoil.mesosphere.com'],
+            timeout=30,
+            stdin=subprocess.DEVNULL)
+    except subprocess.TimeoutExpired:
+        assert False, 'timed out waiting for process to exit'
+
+    assert returncode == 1
+    assert b"'' is not a valid response" in stdout