Unit tests

[juliamourac]

• Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Unit tests

• What is the current behavior? (You can also link to an open issue here)

• What is the new behavior (if this is a feature change)?
  Added unit tests for History and Persister's services, with 75% coverage

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  No

• Is there any issue related to this PR in other repository? (such as dojot/dojot)

• Other information:

[mprevide]

Could you add codecov to the repository? :)
it could be in this same PR.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (development@feb70d1). Click here to learn what that means.
The diff coverage is n/a.

@@              Coverage Diff               @@
##             development      #61   +/-   ##
==============================================
  Coverage               ?   73.62%           
==============================================
  Files                  ?        7           
  Lines                  ?      417           
  Branches               ?        0           
==============================================
  Hits                   ?      307           
  Misses                 ?      110           
  Partials               ?        0

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[mprevide]

Could you remove /tests and setup.py from codecov statistics?
@juliamourac

[mprevide]

Seems ok for me :)