Added regression test for bug #3023 which was solved by Matthieu's
commit r16134 (eta was missing in Flexible/Rigid and
SemiFlexible/Rigid conversion problems).

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16409 85f007b7-540e-0410-9357-904b9bb8a0f7