[test]Reenable test of SubgridL2FunctionalAssembler

* Avoid nasty double assignment `a=b=0`
* Provide `QuadratureRuleKey` with appropriate order to L2-assemblers
* Provide appropriate order to H1-assemblers
* Reenable test of `SubgridL2FunctionalAssembler`
* Reenable test of `SubgridH1FunctionalAssembler` for `UGGrid`
* Avoid short circuit evaluation of test results. This implicitly
  disabled checks if a previous one was failing.

The checks for `ALUGrid<3,3, Dune::simplex, Dune::nonconforming>` are still
disabled, because the failure mentioned in
#14
persists.