NEW: reset global seed before every test case

[martins0n]

IMPORTANT: Please do not create a Pull Request without creating an issue first.

Before submitting (must do checklist)

• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

Type of Change

• Examples / docs / tutorials / contributors update
• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Proposed Changes

The order of test cases is every important in the case of using random.
As the first step of fixing randomness of random - reset of global state for every test case is proposed

Related Issue

Closing issues

[codecov-commenter]

Codecov Report

Merging #155 (97b9eec) into master (ffbc70e) will decrease coverage by 0.03%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #155      +/-   ##
==========================================
- Coverage   87.18%   87.15%   -0.04%     
==========================================
  Files          71       71              
  Lines        3106     3106              
==========================================
- Hits         2708     2707       -1     
- Misses        398      399       +1     

Impacted Files	Coverage Δ
etna/datasets/tsdataset.py	86.43% <33.33%> (-0.51%)	⬇️

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