bt2: Make test_query_executor pass

bt2: Make test_query_executor pass

This patch fixes and enables test_query_executor.py.

The only important change is in the query method: it is necessary to
check for `is_canceled` prior to calling the API function, since it
asserts that the executor is not cancelled.

I have removed the __eq__ method and tests, since it's quite unlikely to
be useful for the QueryExecutor class.

Change-Id: If88c2552627358b06f7d6068126034a20ebf88e8
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1252
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>