LatentFactorMOKernel should be tested by the standard test suit

[david-vicente]

Now that testing methods for MOKernels ara available maybe we can improve the code here: https://github.com/JuliaGaussianProcesses/KernelFunctions.jl/blob/4569859cef362ef3c1fe3848290bfb090a7ee0f1/test/mokernels/slfm.jl#L33