Closes #2102

Describe your changes here:

Related to #1985
The shared memory was often not initialized properly but we did not detect it.

• Merge Bugfix: Ignore warnings when executing CMake tests #1996 first.

I added an error code to t8_shmem_init following the convention of sc_shmem_init.
This actually caused many of our tests to fail giving more reason to this feature.
The errors are fixed with #1996 which should be merged first.

When the error code is met, we currently abort the program.

The reasoning is that shared memory should always be available to us, since it is part of MPI >=3.0

(This creates the new issue of requiring MPI 3.0 in our build system).

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