Fix flaky TestTruncation.truncation_discrete_random

[mattiadg]

Closes #6206

• Add seed to rejection test
• Add a comment about the test probability

What is this PR about?
Fix Potential flaky test: truncation_discrete_random
I add a seed to the test part that can fail stochastically.

Checklist

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Major / Breaking Changes

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Bugfixes / New features

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Docs / Maintenance

• Fix flaky TestTruncation.truncation_discrete_random

[codecov]

Codecov Report

Merging #6229 (f97df7c) into main (7503730) will decrease coverage by 2.82%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6229      +/-   ##
==========================================
- Coverage   93.61%   90.79%   -2.83%     
==========================================
  Files         101      101              
  Lines       22120    22129       +9     
==========================================
- Hits        20707    20091     -616     
- Misses       1413     2038     +625     

Impacted Files	Coverage Δ
pymc/tests/distributions/test_truncated.py	99.48% <100.00%> (ø)
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%> (-28.84%)	⬇️
pymc/sampling.py	71.22% <0.00%> (-11.41%)	⬇️
pymc/backends/report.py	90.00% <0.00%> (-10.00%)	⬇️
pymc/step_methods/arraystep.py	89.36% <0.00%> (-4.97%)	⬇️
pymc/backends/base.py	86.34% <0.00%> (-1.61%)	⬇️
pymc/tests/backends/test_arviz.py	99.02% <0.00%> (+0.01%)	⬆️
... and 2 more