Random failures of both `umfpacksolvertest` and `cholmodsolvertest` on `git-debian-11-gcc-9-20`

For some time both test fail for no obvious reason. I am not able to reproduce it locally.

I struggled to narrow down the root cause with a lot of different CI configs in this MR (!62 (merged)) and also in my fork (https://git.imp.fu-berlin.de/patrick.jaap_at_tu-dresden.de/dune-solvers/-/pipelines)

The worst observation was that on the same commit the test passed once and failed once https://git.imp.fu-berlin.de/agnumpde/dune-solvers/-/jobs/171102 vs https://git.imp.fu-berlin.de/agnumpde/dune-solvers/-/jobs/171173

The basic analysis tools for undefined behavior (valgrid,gcc flags) reported no problems.