Allows to define testing specification via `TESTBENCH_SPECIFICATION` environment variable

[crynobone]

I always wanted to be able to run tests in different specifications such as with and without immutable date.

      - name: Execute tests
        run: vendor/bin/phpunit

      - name: Execute tests (Immutable Dates)
        run: vendor/bin/phpunit
        env:
          TESTBENCH_SPECIFICATION: 'immutable-date'