[TM-4] Add a test-suite

Description

Problem: current implementation doesn't have automatic testing, it has some functions to run from REPL to check, if the contract in the file being parsed successfully.

Solution: add unit test using HSpec which automatically runs parser on files from contracts/ and check if they were parsed successfully.

Related issue(s)

https://issues.serokell.io/issue/TM-4

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests

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

• Documentation

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

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