tests: bt2: add `--test-case` argument to testrunner.py

tests: bt2: add `--test-case` argument to testrunner.py

The `-t/--test-case` argument allows to specify the exact test case to
run.

Make the `--test-case` and the existing `--pattern` options mutually
exclusive and require that one of the two is provided.

It can be used like this:
  ./tests/utils/run_python_bt2 python3 ./tests/utils/python/testrunner.py \
    -t test_event.EventTestCase.test_clock_value \
    ./tests/bindings/python/bt2

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Icc9204d05bac4cef9cc509f2b19a9e515b0a40e7
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1612
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>