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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.13%. Comparing base (cb24b07) to head (75c02b4).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1827      +/-   ##
==========================================
+ Coverage   88.12%   88.13%   +0.01%     
==========================================
  Files         119      119              
  Lines       29980    29980              
==========================================
+ Hits        26421    26424       +3     
+ Misses       3559     3556       -3     

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[fingolfin]

This seems to be ready now.