test_git counts only its own ssh-agents (#172)

The unit tier runs under xdist, so another worker's push could start an
ssh-agent between a test's count and its check. The test sshd now tags
every session with NISSE_TEST_SSHD=<its root>, and the tests count only
agents carrying their own sshd's tag. A new test starts a foreign agent
alongside and checks it isn't counted.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J5Utyfzw7gF7UhjRDEtjTk

nisse-core/src/nisse_core/execution/testing.py

@@ -18,6 +18,21 @@ from nisse_core.execution.ssh import SSHHost
 
 SSHD = "/usr/sbin/sshd"
 
+# Set in every session of a local sshd, to its root directory.
+SESSION_MARK = "NISSE_TEST_SSHD"
+
+
+def started_by(host: SSHHost, pid: str) -> bool:
+    """Whether process ``pid`` was started in a session of ``host``'s local
+    sshd: it carries that sshd's SESSION_MARK. A process that's gone, or
+    another user's, wasn't."""
+    mark = f"{SESSION_MARK}={host._key.parent}".encode()  # the sshd root
+    try:
+        environ = Path(f"/proc/{pid}/environ").read_bytes()
+    except OSError:
+        return False
+    return mark in environ.split(b"\0")
+
 
 def _free_port() -> int:
     with socket.socket() as sock:
@@ -61,7 +76,12 @@ def local_sshd(root: Path) -> Iterator[SSHHost]:
                 "MaxStartups 50",
                 # Stands in for a provisioned build VM: sessions see the test
                 # runner's PATH, so its Python and tools are "installed".
-                f"SetEnv PATH={os.environ.get('PATH', '/usr/bin:/bin')}",
+                # SESSION_MARK tags every process this sshd starts, so a test
+                # can tell its own from another worker's (#172).
+                (
+                    f"SetEnv PATH={os.environ.get('PATH', '/usr/bin:/bin')} "
+                    f"{SESSION_MARK}={root}"
+                ),
                 "",
             ]
         )