Cover module with tests [#3401230]

We need to create tests to check the work of the Drush command that we use in the module.

I guess it should be smth like:
- create a new vocabulary;
- add a few items to the vocabulary;
- run the drush command;
- check the count of terms in the vocabulary (should be 0).

Issue fork taxonomy_delete-3401230

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3401230-cover-module-with changes, plain diff MR !17
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