Add tests for some modules

[chshersh]

For example, Serokell.Util.Common .

UPDATE:

Tests for modules such as Serokell.Util.Common and Serokell.Data.Variant.Variant have been added. The only remaining thing is to add tests for Text show/read functions in Serokell.Util.Text for non-'Decimal' types - issues with precision remain that make it difficult to simply test for 'read . show = id'.