Review iota-graphql test coverage

[samuel-rufi]

Description of change

Reviews the test coverage for iota-graphql-rpc and iota-graphql-e2e-tests.

Links to any relevant issues

Fixes #1562

Type of change

• Documentation

How the change has been tested

cargo nextest run --features pg_integration --no-fail-fast --test-threads 1

Change checklist

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked that new and existing unit tests pass locally with my changes

[tomxey]

lgtm 🌄