Tests do not consistently respect passed error tolerance

Currently the tests library cannot be used with DFTK because the error tolerances are not consistently applied. For example 

https://github.com/JuliaMolSim/AtomsCalculators.jl/blob/master/src/submodules/AtomsCalculatorsTesting.jl#L46

fails for crude numerical parameters in DFT calculations. We should make a pass on that.