tests/utils/python/tjson.py: print actual type when a type error occurs

tests/utils/python/tjson.py: print actual type when a type error occurs

Make `_check_type` output the actual type in addition to the expected
type.  Example:

    Before: # TypeError: `.[0]."cond-id"`: expecting a string value
    After:  # TypeError: `.[0]."cond-id"`: expecting a string value, got an array

Change-Id: I9615aa380b12fe5e7fd65b079840b779f9530f3a
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11781
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>