Unit tests fail depending on architecture

[s-mayani]

Some unit tests fail because the tolerances/definitions of types vary between different computing systems.
In particular, the following:

• unit_tests/FEM/AssembleRHS.cpp passes on Alps but fails at Jülich (CI system).
• FFT unit tests, see issue FFT unit tests fail #353

Feel free to edit this issue to add more occurrences.