More generic test_elem methods

[fingolfin]

Also make sure other packages which only load Rings-conformance-tests.jl
get them.

Finally cleanup how we include Groups-conformance-tests.jl

Unfortunately this causes test failures and eventually a bus error crash (!). This is due to the test_elem(Rx::AbstractAlgebra.SeriesRing) method. The crash can be triggered with

R, x = power_series_ring(ZZ, 10, :x)
include("test/Rings-conformance-tests.jl");
test_Ring_interface(R)

Note that there are failures before that because the tests for the unsafe ops add! etc. don't take into account that the series rings are inexact. I hope #1816 will fix that part.

But even taking that into account, we really shouldn't get a hard crash here

[fingolfin]

@joschmitt figured out the cause for the crash (to be fixed by PR #1828), which in turn was triggered by the coefficients lists for the series starting with a zero.

The failures in the unsafe ops tests are still there, though.