tests/src.ctf.fs: add CTF 2 tests for no stream class, no trace class

tests/src.ctf.fs: add CTF 2 tests for no stream class, no trace class

During development, we hit some crashes when the trace metadata did not
specify any data stream class fragment (with and without an explicit
trace class fragment).  Add tests for that.

Change-Id: Ic8ce4c7a2ae78b2f484a86e5b1306b7eae606528
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12751
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>