gplugin/gplugin

Update test-loader-registration to use a new instance of GPluginManager

This avoids using private API and fixes the build on Windows.

Testing Done:
Ran the tests under an msys2 mingw64 shell without issue.

Reviewed at https://reviews.imfreedom.org/r/3025/
The following individuals have contributed to this project.
Ankit Vani <a@nevitus.org>
EionRobb <eion@robbmob.com>
Elliott Sales de Andrade <quantum.analyst@gmail.com>
Gary Kramlich <grim@reaperworld.com>
Jason Scherer <schereja@gmail.com>
John Bailey <rekkanoryo@rekkanoryo.org>
Mike Ruprecht <cmaiku@gmail.com>
Olaf Hering <olaf@aepfle.de>
Paul Jimenez <pj@place.org>
Richard Laager <rlaager@pidgin.im>
Sorvival <sorvival@tutanota.com>
ivanhoe <ivanhoe@fiscari.de>