bridge_softmix: Fix sfu_append_source_streams test.

* validate_stream: zero result from ast_format_cap_identical indicates
  they are not identical, rather than non-zero indicating an error.
* validate_original_streams: use num_streams instead of
  ARRAY_LEN(params).
* Fix declaration of alice_dest_stream and bob_dest_stream.

Change-Id: I6b1dd8bed10439d3c7406f033eb1896b6c419147