[#17] Automate the parser tests

[YuriRomanowski]

Description

Problem: Some example tests have been written for time references parser, but they are not checked automatically.

Solution: Use doctest-parallel to automate doctests.

Related issue(s)

Fixed #17

:white_check_mark: Checklist for your Pull Request

Related changes (conditional)

• Tests

  • [x] If I added new functionality, I added tests covering it.
  • [x] If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • [x] I checked whether I should update the docs and did so if necessary:
  • README
  • Haddock

Stylistic guide (mandatory)

• [x] My commits comply with the policy used in Serokell.
• [x] My code complies with the style guide.