test `merge_fields_not_in_interface` is flaky

[leoyvens]

It frequently fails on CI with:

thread 'merge_fields_not_in_interface' panicked at 'called `Result::unwrap()` on an `Err` value: Unknown(NotFound)',
/home/runner/work/graph-node/graph-node/store/test-store/src/store.rs:196:54

[leoyvens]

Another flaky test result I got locally (reproduced only once):

thread 'fragments_merge_selections' panicked at 'failed to convert to modifications: StoreError(CloneableAnyhowError(store error: relation "sgd366.child" does not exist))',
<path>/graph-node/store/test-store/src/store.rs:296:10