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