Add more and generic test_elem methods

[fingolfin]

In order to run ring conformance tests, one has to provide a suitable test_elem method.

I think we should provide more generic ones for those, e.g. for polynomial and series ring. To this end we can make them recursive.

For example, we currently have

function test_elem(R::AbstractAlgebra.Rationals{BigInt})
   n = big(2)^rand(1:100)
   m = big(2)^rand(1:100)
   return rand(ZZ, -n:n)//rand(ZZ, 1:m)
end

which we could change to

function test_elem(R::AbstractAlgebra.Rationals)
   B = base_ring(R)
   return test_elem(B)//test_elem(B)
end

My motivation is that I wanted to add some conformance tests in Nemo so that the various mistakes I made in Nemocas/Nemo.jl#1869 (which @lgoettgens spotted) would be found by the new tests in #1816.

That requires me to write tests for a bunch of relative and absolute power series rings, and to me it seems this really are always the same...

(BTW I wonder if we should look into replacing our crude test_elem system with something more sophisticated like https://github.com/Seelengrab/Supposition.jl which implements "shrinking" to find minimal reproducers of issues which is really nice)