Make test_ts optional in plot_forecast

[Mr-Geekman]

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

Look at #314.

Related Issue

#314.

Closing issues

Closes #314.

[codecov-commenter]

Codecov Report

Merging #321 (bf72243) into master (a764443) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #321      +/-   ##
==========================================
- Coverage   54.42%   54.38%   -0.05%     
==========================================
  Files          97       97              
  Lines        4777     4781       +4     
==========================================
  Hits         2600     2600              
- Misses       2177     2181       +4     

Impacted Files	Coverage Δ
etna/analysis/plotters.py	12.16% <0.00%> (-0.27%)	⬇️

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