Restructure test files and extend usage of (corrected) pytest fixtures

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In #18 / #227 a series of `@pytest.fixture` were created in `test_update_recipe_file.py`.

These should be moved to a new `conftest.py` file for reuse per the new [wiki: Testing requirements](https://github.com/MetOffice/CMEW/wiki/Developer-Guide#testing-requirements).

Fixture reuse should be possible in `test_create_request_file.py`.

The fixture `env-var` perhaps also be expanded as a single source of truth for mocking environment variables.

Comment from #227 from @ehogan:

> If you decide to move these tests as per the newly created and discussed [wiki: Testing requirements](https://github.com/MetOffice/CMEW/wiki/Developer-Guide#testing-requirements), these fixtures could be added to a [conftest.py](https://docs.pytest.org/en/7.1.x/reference/fixtures.html#conftest-py-sharing-fixtures-across-multiple-files) file in the app/unittest/tests/ directory! :)

Start with the fixtures in test_update_recipe_file.py and the related tests in test_create_request_file.py, then review the Testing requirements guidance and pytest's conftest.py fixture rules. Move reusable fixtures into conftest.py under app/unittest/tests/ and verify both test files reuse them. Confirm that env-var provides the intended single source for mocked environment variables and that the affected tests pass.

Written by the indexing model from the issue text.