[test][tracer][multidoamain] Use smaller epsilon

This was previously not possible because of the bug in the
explicit/numdiff assembler. The large epsilon masked the error
in the storage derivative. Now it also works with small epsilon.

test/multidomain/facet/tracer_tracer/params.input

@@ -27,4 +27,4 @@ SpatialParams.Porosity = 0.9
 OutputName = test_md_facet_tracertracer
 
 [Assembly]
-NumericDifference.BaseEpsilon = 1e10
+NumericDifference.BaseEpsilon = 1e-10