tests: add python plugin provider path to lib search path

tests: add python plugin provider path to lib search path

I am working on a test that uses a component class implemented in a
Python plugin.  For it to work, babeltrace therefore needs to be able to
load the python plugin provider lib.  It currently isn't found:

    LIB/PLUGIN init_python_plugin_provider@plugin.c:84 Loading Python plugin provider module.
    LIB/PLUGIN init_python_plugin_provider@plugin.c:94 Cannot open `libbabeltrace2-python-plugin-provider.so`: libbabeltrace2-python-plugin     -provider.so: cannot open shared object file: No such file or directory: continuing without Python plugin support.

This patch makes run_python_bt2 add the right path to LD_LIBRARY_PATH
(PATH on Windows) so that babeltrace can load python plugins.

With this added we don't need the feature of run_python_bt2 and
run_python_bt2 that allowed to append an additional directory to
LD_LIBRARY_PATH, so it is removed.

Change-Id: Ie72f59c9ef4866c7f0eae0e6f49025d2fa1ba127
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1608
CI-Build: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>