Move pytest fixture test to .github folder (#6921)

The pytest fixture tests need to be isolated from the regular test suite to
verify that they can function independently of the aiida-core test suite.
Including them in the main test suite risks inadvertently reusing
configurations, which previously led to bug PR #6904 fixed.

However, moving these tests to the `src` would result in them being included in
distribution packages, which is undesirable. Therefore, we relocate them to the
`.github` directory, ensuring they remain part of the repository but are
excluded from the shipped package.

Author: agoscinski

src/aiida/manage/tests/test_pytest_fixtures.py renamed to .github/test_legacy_pytest_fixtures.py

@@ -175,7 +175,7 @@ jobs:
         extras: tests
 
     - name: Test legacy pytest fixtures
-      run: pytest --cov aiida --noconftest src/aiida/manage/tests/test_pytest_fixtures.py
+      run: pytest --cov aiida --noconftest .github/test_legacy_pytest_fixtures.py
 
     - name: Upload coverage report
       if: github.repository == 'aiidateam/aiida-core'