bt2: add `StringValue.__contains__()` method

bt2: add `StringValue.__contains__()` method

This is useful to use the `in` Python operator like we do with regular
Python `str` objects:

  if 'my_env_var' in env_string_value:
    ...

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I8f7f5c02d71bc089e14c3b8f2ffa81045a9e8f15
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1954
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>