tests: fix Python plugin provider tests on Windows

tests: fix Python plugin provider tests on Windows

On Windows, the embedded Python interpreter in the Python plugin
provider can't automatically locate the path to it's own modules. Set
the PYTHONHOME variable to the proper value in 'run_python_bt2()'.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Change-Id: Ie30ac3212a208d77654add78604c534c05efe518
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1748
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>