fix(test): pin stdin so the net-scenario harness stops hanging

The stubbed ssh is `cat >/dev/null`, which drains stdin to EOF. Without an explicit stdin the stub inherits whatever the test runner had, so it returned instantly when stdin was redirected and blocked forever when it was a terminal or a live pipe. All five tests then hit their 20-second timeout.

That made the suite unreliable in the case that matters most, running it by hand. `make test-unit </dev/null` passed while the same target run from a terminal failed, which reads as a flaky test rather than an environment-dependent one.

1 files changed, 6 insertions, 0 deletions

diff --git a/tests/net-scenarios/test_run_net_scenarios.py b/tests/net-scenarios/test_run_net_scenarios.py
index 1d92185..a9cd275 100644
--- a/tests/net-scenarios/test_run_net_scenarios.py
+++ b/tests/net-scenarios/test_run_net_scenarios.py
@@ -63,6 +63,12 @@ class RunNetScenarios(unittest.TestCase):
         return subprocess.run(
             ["bash", SCRIPT, "--target", "root@fake-vm"],
             capture_output=True, text=True, timeout=20, env=env,
+            # The stubbed ssh is `cat >/dev/null`, which drains stdin to EOF.
+            # Without this the stub inherits whatever stdin the test runner
+            # had, so `make test-unit` passed when stdin was redirected and
+            # hung on all five tests when it was a terminal or a live pipe --
+            # which is how it gets run by hand.
+            stdin=subprocess.DEVNULL,
         )
 
     def test_all_checks_pass_exits_zero(self):