[WIP] Save timeseries node: added tests

[irynamatveieva]

Pull Request description

Added tests for save timeseries node.

General checklist

• You have reviewed the guidelines document.
• Labels that classify your pull request have been added.
• The milestone is specified and corresponds to fix version.
• Description references specific issue.
• Description contains human-readable scope of changes.
• Description contains brief notes about what needs to be added to the documentation.
• No merge conflicts, commented blocks of code, code formatting issues.
• Changes are backward compatible or upgrade script is provided.
• Similar PR is opened for PE version to simplify merge. Crosslinks between PRs added. Required for internal contributors only.

Back-End feature checklist

• Added corresponding unit and/or integration test(s). Provide written explanation in the PR description if you have failed to add tests.
• If new dependency was added: the dependency tree is checked for conflicts.
• If new service was added: the service is marked with corresponding @TbCoreComponent, @TbRuleEngineComponent, @TbTransportComponent, etc.
• If new REST API was added: the RestClient.java was updated, issue for Python REST client is created.
• If new yml property was added: make sure a description is added (above or near the property).