test_plugin.py: do not search in user/system directories for tests

test_plugin.py: do not search in user/system directories for tests

In `test_plugin.py`, make bt2.find_plugin() not search in user and
system directories so that it needs to find the specific plugin in the
test's search path (given by `BABELTRACE_PLUGIN_PATH`) or fail.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I9bad521d079ce602fa152d9e24ab6b145e8edce9
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1880
Tested-by: jenkins <jenkins@lttng.org>