test: lock speechbrain lengths and identity camera filter

Filter --cameras 1,5 by cam-NN identity instead of discovery index.

discovery.py

@@ -34,6 +34,14 @@ class Camera:
     extra_video_devices: list[str] = field(default_factory=list)
 
 
+def select_cameras(cameras: list[Camera], spec: list[str]) -> list[Camera]:
+    """`--cameras 1,5` means identities cam-01 and cam-05, not discovery index."""
+    if spec == ["all"]:
+        return list(cameras)
+    wanted = {f"cam-{int(i):02d}" for i in spec}
+    return [c for c in cameras if c.identity in wanted]
+
+
 def _run(cmd: list[str]) -> str:
     return subprocess.run(cmd, capture_output=True, text=True, check=False).stdout