tests: use os.pathsep when appending to BABELTRACE_PLUGIN_PATH

tests: use os.pathsep when appending to BABELTRACE_PLUGIN_PATH

On Windows, we expect BABELTRACE_PLUGIN_PATH entries to be separated
with ';', not ':' (like PATH).  Python conveniently provides an
OS-dependent definition for this.

Change-Id: Ie53af04e933258963b8495c72e7607a623e83ddb
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1883
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>