Add stream class's single clock class tests

Add stream class's single clock class tests

Those new tests verify that:

* You cannot have a clock class in one of the stream class's field types
  and a different one in one of its event classes.
* You cannot have two different clock classes in two different event
  classes of the same event class.
* You cannot set a stream class's CTF writer clock and have a clock
  class within this stream class which is not the stream class's clock's
  class.

The tests check those conditions when calling specific API functions and
in the expected order:

* bt_trace_add_stream_class()
* bt_event_create()
* bt_stream_class_add_event_class()

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>