README: update with instructions on running tests

Dependencies

• [2] 43CQEJTE README.md update
• [*] NVMDS7GE Add README.md
• [*] LA5L7JCU README.md: info about runHydra/foreman
• [*] LX326CB7 Add hydra OpenAPI description (#750)

Change contents

• edit in README.md at line 108
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  [5.977]
  +

  + ### Running Tests
• edit in README.md at line 111
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  + After making your changes, verify the test suite still passes. After following the steps in [Development Environment](#development-environment), run:

  +

  + ```

  + $ nix-shell

  + $ make check

  + ```

  +