[devtools] vreg becomes a class ppty
After multiple devtools refactoring, the vreg used for test is now *always* got
from the repository. Making this explicit is a step toward simplification of the
vreg handling during various initialisation phases.
Related to #2944813
doc/book usr/share/doc/cubicweb-documentation
doc/html usr/share/doc/cubicweb-documentation
debian/cubicweb-doc usr/share/doc-base/cubicweb-doc