test: lock speechbrain lengths and identity camera filter

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

tests/test_config.py

@@ -0,0 +1,7 @@
+from config import _parse_cameras
+
+
+def test_parse_cameras_list():
+    assert _parse_cameras("1,5") == ["1", "5"]
+    assert _parse_cameras("all") == ["all"]
+    assert _parse_cameras("1-3") == ["1", "2", "3"]