Don't run moebius_3 test when available memory is limited

[tueda]

Here, I simply set constant memory requirements. (Maybe this is enough?) In principle, one can consider the number of cpus etc. but it may be too complicated.

The serial version requires 8 GB (in SI, not GiB), so 8 GiB system memory is enough to run it. The threaded/mpi versions need 20GB. Note that 16 GiB is available on GitHub-hosted runners.

[jodavies]

This seems good to merge to me.

[Korg666999]

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