test_plugin: use env. var. instead of global symbol because G_MODULE_BIND_LOCAL

Share data between test plugins and the plugin test with environment
variables instead of using global symbols because shared objects are
opened with G_MODULE_BIND_LOCAL now.

Also add BT_PLUGIN_MODULE() to test plugins so that their special
sections exist and contain entries.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>