Fix flaky TestMixture.test_component_choice_random

[bherwerth]

What is this PR about?
Fixes #6210:

• Use a seed for taking draws
• Increase number of draws to 20

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? (does not apply)
• Fill out the short summary sections 👇

Major / Breaking Changes

• ...

Bugfixes / New features

• ...

Docs / Maintenance

• Fix flaky TestMixture.test_component_choice_random

[codecov]

Codecov Report

Merging #6222 (514f765) into main (7503730) will decrease coverage by 18.74%.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6222       +/-   ##
===========================================
- Coverage   93.61%   74.86%   -18.75%     
===========================================
  Files         101      101               
  Lines       22120    22129        +9     
===========================================
- Hits        20707    16567     -4140     
- Misses       1413     5562     +4149     

Impacted Files	Coverage Δ
pymc/tests/distributions/test_mixture.py	99.35% <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/tests/distributions/test_continuous.py	0.00% <0.00%> (-99.77%)	⬇️
pymc/tests/distributions/test_multivariate.py	0.00% <0.00%> (-99.45%)	⬇️
pymc/tests/distributions/test_discrete.py	0.00% <0.00%> (-98.89%)	⬇️
pymc/tests/distributions/test_distribution.py	0.00% <0.00%> (-97.93%)	⬇️
pymc/tests/distributions/util.py	43.70% <0.00%> (-47.17%)	⬇️
pymc/distributions/multivariate.py	54.86% <0.00%> (-37.39%)	⬇️
... and 19 more

[ricardoV94]

Looks great, but I would rather keep the old check than the new one. What do you think?

[ricardoV94]

Thanks @bherwerth!