Create __init__.py for tests

[ferrine]

Without __init__.py this module is not installed in pymc-experimental and test builds are broken.

What is this PR about?
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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?
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Major / Breaking Changes

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[codecov]

Codecov Report

Merging #6104 (19b8dde) into main (23df308) will increase coverage by 2.51%.
The diff coverage is 97.95%.

@@            Coverage Diff             @@
##             main    #6104      +/-   ##
==========================================
+ Coverage   89.56%   92.07%   +2.51%     
==========================================
  Files          72       86      +14     
  Lines       12927    17861    +4934     
==========================================
+ Hits        11578    16446    +4868     
- Misses       1349     1415      +66     

Impacted Files	Coverage Δ
pymc/distributions/__init__.py	100.00% <ø> (ø)
pymc/sampling.py	82.13% <ø> (-0.02%)	⬇️
pymc/tests/distributions/test_dist_math.py	99.25% <ø> (ø)
pymc/tests/distributions/test_timeseries.py	93.75% <90.00%> (ø)
pymc/tests/distributions/util.py	90.86% <90.86%> (ø)
pymc/printing.py	86.08% <92.59%> (-0.58%)	⬇️
pymc/distributions/timeseries.py	78.52% <94.25%> (+0.53%)	⬆️
pymc/distributions/logprob.py	97.31% <94.73%> (-0.42%)	⬇️
pymc/aesaraf.py	92.91% <96.00%> (+0.71%)	⬆️
pymc/distributions/multivariate.py	92.31% <96.20%> (+0.07%)	⬆️
... and 29 more

[ricardoV94]

CC @Armavica, we should keep this in mind in future PRs