Nested Create: Integration/Unit testing

[ayush3797]

For every sub-task being merged into the parent feature branch for nested insertions: dev/NestedMutations, add unit/integration test wherever possible and required.

PRs to be merged into the parent branch:

1. Multiple-create: Schema generation #1902
2. Multiple-create: Authorization  #1943
3. Multiple Create - Database Query Generation and Selection Set Resolution #1994
4. Feature Flag for Nested Mutations: CLI changes #1983
5. Adding validation to disallow multiple relationships between same source and target entities #1929
6. Removing wrong foreign key definitions #1997

As and when more sub-tasks are created, need to append them to the list.

[ayush3797]

Adding unit/integration tests to the individual PRs itself. Hence no need to track via this issue.