[hooks] build EnsureSymmetricRelations{Add,Delete} dynamically Instead of setting the __select__ attribute at registration time, which confuses the filterable_selectors cache when running different tests (with different schemas) in the same process, just make entirely new classes. Closes #8785708.

[qunit] stop dealing with filesystem paths qunit tests need a few things served by cubicweb: - qunit itself, which is now handled by CWDevtoolsStaticController (serving files in cubicweb/devtools/data) - standard cubicweb or cubes data files, handled by the DataController - the tests themselves and their dependencies. These can live in <apphome>/data or <apphome>/static and be served by one of the STATIC_CONTROLLERS This avoids having to guess in CWSoftwareRootStaticController where to serve things from (some files may be installed, others are in the source tree), and should hopefully make it possible to have these tests pass when using tox, and to write qunit tests for cubes, outside of cubicweb itself. This requires modifying the tests to only declare URL paths instead of filesystem paths, and moving support files below test/data/static.