[maint] Reduce likelihood of creating dependent tests from fixtures

[PGijsbers]

Change

Restructure the fixtures around the default json objects to reduce the chance of accidentally creating dependent tests. In the current situation:

• some of the fixtures are session scoped, which means the same result is used for multiple tests, but
• that result can be modified since the dictionary is mutable

This means that if you're not careful, you can end up modifying the state for other tests.
Now, we separate out loading from disk (once per session) to creating the dictionary (once per test), removing the ability to persist state across tests. Note that a single test can still (indirectly) request the same fixture twice, in which case there still needs to be a precaution not to modify the dicts directly.

How to Test

Run the tests, think things through. Alternatively, the following tests will pass on develop but not on this branch:

def test_a(body_concept):
    body_concept["marker"] = 1  # marker is not normally present on the body_concept fixture
    
def test_b(body_concept):
    assert "marker" in body_concept

if test_a runs before test_b, then test_b will pass (on develop). If test_a isn't run, or run after test_b, then it fails (on develop). On this branch test_b will always fail, since body_concept is now a function-level fixture.

Checklist

• Tests have been added or updated to reflect the changes, or their absence is explicitly explained.
• Documentation has been added or updated to reflect the changes, or their absence is explicitly explained.
• A self-review has been conducted checking:
  • No unintended changes have been committed.
  • The changes in isolation seem reasonable.
  • Anything that may be odd or unintuitive is provided with a GitHub comment explaining it (but consider if this should not be a code comment or in the documentation instead).
• All CI checks pass before pinging a reviewer, or provide an explanation if they do not.

Related Issues

[PGijsbers]

This deep copy ensures that the loaded json that persists for the session is never edited.
The fact that this is now a function-level fixture in turn means that if the return value of body_concept does get modified, it does not carry over to other tests ( but may carry over to other fixtures which depend on it in the same test).

[PGijsbers]

The reason that we use deepcopy here, is that otherwise we would edit body_concept directly.
While this would no longer carry over to other tests, it could carry over to other fixtures in the same test.
E.g., if body_resource and contact would both be requested by the same test, then we need to avoid that either of those fixtures is directly modifying the body_concept since it is still persistent within the same test.