LocalizedEuclideanRing: add tests, fix some bugs, run conformance tests

[fingolfin]

Based on Nemo's test/HeckeMiscLocalization-test.jl

As always (?), running conformance tests revealed a genuine bug (this time in is_unit(::LocalizedEuclideanRingElem)

[codecov]

Codecov Report

❌ Patch coverage is 57.89474% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.46%. Comparing base (0eb3ed5) to head (3d2c79c).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/generic/Misc/Localization.jl	57.89%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2254      +/-   ##
==========================================
+ Coverage   88.07%   88.46%   +0.38%     
==========================================
  Files         126      126              
  Lines       31728    31994     +266     
==========================================
+ Hits        27946    28305     +359     
+ Misses       3782     3689      -93     

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