Move LensKit test support into lenskit.testing package

[mdekstrand]

This creates a new package, lenskit.testing, and moves the test support code from lenskit.util.test into here.

Work on #574 will live in the new package.

[codecov]

Codecov Report

Attention: Patch coverage is 85.13514% with 22 lines in your changes missing coverage. Please review.

Project coverage is 90.24%. Comparing base (a01e859) to head (a7f7275).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
lenskit/lenskit/testing/_arrays.py	68.75%	20 Missing ⚠️
lenskit/lenskit/testing/__init__.py	92.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #575      +/-   ##
==========================================
+ Coverage   90.20%   90.24%   +0.03%     
==========================================
  Files         103      105       +2     
  Lines        6403     6429      +26     
==========================================
+ Hits         5776     5802      +26     
  Misses        627      627              

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