Fixes for SparsePolyRing

[lgoettgens]

Contains and thus closes #1691.

Contents:

• Fixes two of the three issues experienced in Add conformance tests for Generic.SparsePoly #1689
• Fixes some ambiguities that arise in running the tests
• Removes the export SparsePolynomialRing (technically breaking)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 23 lines in your changes are missing coverage. Please review.

Project coverage is 85.91%. Comparing base (9c828f7) to head (89003c5).
Report is 1 commits behind head on master.

Files	Patch %	Lines
src/generic/SparsePoly.jl	0.00%	23 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1692      +/-   ##
==========================================
- Coverage   87.34%   85.91%   -1.44%     
==========================================
  Files         116      116              
  Lines       29713    29736      +23     
==========================================
- Hits        25954    25548     -406     
- Misses       3759     4188     +429     

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