Add conformance tests for is_unit & is_nilpotent

[fingolfin]

Also improve is_nilpotent for matrices / matrix ring elements to work if the base ring is trivial.

Resolves #1941

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.17%. Comparing base (3e14b81) to head (8d86cbd).
⚠️ Report is 42 commits behind head on master.

Files with missing lines	Patch %	Lines
ext/TestExt/Rings-conformance-tests.jl	89.28%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1947   +/-   ##
=======================================
  Coverage   88.17%   88.17%           
=======================================
  Files         126      126           
  Lines       32073    32098   +25     
=======================================
+ Hits        28279    28302   +23     
- Misses       3794     3796    +2     

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[fingolfin]

Test fails:

  Expression: is_unit(a) == is_unit(a ^ 2)
  function is_unit is not implemented for argument
  PBWAlgQuoElem{QQFieldElem, Singular.n_Q}: 1//3*a^2*h^2*f^3*e^3 + 3//2*a^3*h^2*f^2*e^3 + 1//2*a^2*h^3*f*e^2

Perhaps someone (@JohnAAbbott ?) can look into adding that method next year.

In the meantime we could make the test conditional here and skip it if a not-implemented-exception is thrown...

[fingolfin]

@fieker @thofma @lgoettgens just talking to @JohnAAbbott whether we can implement is_unit for PBWAlgQuoElem and, well, it is non-trivial.

So we probably should not insist on this being available. But what would be a good strategy to have such tests (they are useful, I found several bugs in our current code with them!) but allow some rings to opt-out of them because they don't implement a certain feature?