[V3] Make pytest fixtures available as a plugin (#1858)

* Move all fixtures to pytest plugin folder

* Add core dunder all

* Update other dunder all's

* Black reformat

redbot/pytest/dataconverter.py

@@ -0,0 +1,12 @@
+from pathlib import Path
+
+from redbot.cogs.dataconverter import core_specs
+
+__all__ = ["get_specresolver"]
+
+
+def get_specresolver(path):
+    here = Path(path)
+
+    resolver = core_specs.SpecResolver(here.parent)
+    return resolver