[cord] Improve test Coverage in cord

[NiranjanAP]

Need comprehensive test coverage to be implemented for all modules of cord. Cover all flows and error conditions

[amarts]

To elaborate more, as of now, when the command cargo test --release --locked --features=runtime-benchmarks --no-fail-fast --verbose --color always --all --all-targets is run in the cord repo, it runs most of the tests. But idea is, there should be a test for making sure every type of error defined in the pallet is covered. That way, developers are sure the expected case for particular Error is met, and is behaving same across.

This helps in upgrade confidence.