Seed pm.sample in BaseSampler(SeededTest) to make deriving test classes deterministic

[mattiadg]

What is this PR about?
Add a seed to pm.sample() in BaseSampler, which is a SeededTest but its draws are not yet seeded.
This solves flaky tests like the one reported in #6246. Closes #6246
Given the complex structure of the tests in this file, made of many mixins of fixtures, each with its own class hierarchies, I found BaseSampler to be the only point where the seed can be added meaningfully.

Checklist

• Explain important implementation details 👆
• Make sure that the pre-commit linting/style checks pass.
• Link relevant issues (preferably in nice commit messages)
• Are the changes covered by tests and docstrings?
• Fill out the short summary sections 👇

Docs / Maintenance

• Seed BaseSampler class tests

[codecov]

Codecov Report

Merging #6251 (83e29a7) into main (3919ed3) will increase coverage by 30.62%.
The diff coverage is n/a.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6251       +/-   ##
===========================================
+ Coverage   60.33%   90.96%   +30.62%     
===========================================
  Files         101      101               
  Lines       22235    22232        -3     
===========================================
+ Hits        13416    20223     +6807     
+ Misses       8819     2009     -6810     

Impacted Files	Coverage Δ
pymc/tests/step_methods/test_slicer.py	0.00% <0.00%> (-100.00%)	⬇️
pymc/tests/step_methods/hmc/test_nuts.py	0.00% <0.00%> (-100.00%)	⬇️
pymc/tests/step_methods/test_metropolis.py	0.00% <0.00%> (-100.00%)	⬇️
pymc/step_methods/metropolis.py	54.56% <0.00%> (-28.43%)	⬇️
pymc/sampling.py	71.14% <0.00%> (-11.05%)	⬇️
pymc/backends/report.py	90.00% <0.00%> (-10.00%)	⬇️
pymc/step_methods/arraystep.py	89.36% <0.00%> (-4.97%)	⬇️
pymc/distributions/multivariate.py	92.32% <0.00%> (+0.10%)	⬆️
pymc/model.py	89.47% <0.00%> (+0.55%)	⬆️
pymc/tests/distributions/util.py	90.86% <0.00%> (+2.46%)	⬆️
... and 62 more

[ricardoV94]

Thanks @mattiadg