.gitlab-ci.yml: test with dune-parmg installed and other configurations

Using the 2.6 branch for dune-fufem modules fails for two reasons:

• The master branch of dune-elasticity is needed (not too complicated to change).
• dune-fufem's releases/2.6-1 branch doesn't work as it misses a backport of (at least) https://git.imp.fu-berlin.de/agnumpde/dune-fufem/merge_requests/44

merged

mentioned in commit d85ff01d