Isolate nested IPC sockets and define session ownership policy

scripts/tests/session_socket.py

@@ -0,0 +1,17 @@
+"""Find the compositor endpoint in an isolated test runtime."""
+from pathlib import Path
+
+
+def ipc_socket(runtime):
+    runtime = Path(runtime)
+    default = runtime / "ferese/control.sock"
+    instances = sorted(path for path in runtime.glob("ferese/instances/*/control.sock") if path.is_socket())
+    if default.is_socket():
+        instances.append(default)
+    if len(instances) > 1:
+        raise RuntimeError("Isolated test runtime contains multiple compositor instances")
+    return instances[0] if instances else default
+
+
+def ipc_environment(environment):
+    return dict(environment, FERESE_SOCKET=str(ipc_socket(environment["XDG_RUNTIME_DIR"])))