Improve debugging by using #[track_caller] in system assert_last_event and assert_has_event

[gui1117]

Without track caller the error message of the assert points to the assert_last_event function, which is not useful.

thread 'tests::set_metadata_works' panicked at /home/gui/Developpement/polkadot-sdk/substrate/frame/system/src/lib.rs:2034:9:
assertion `left == right` failed: expected event RuntimeEvent::Referenda(Event::MetadataSet { index: 0, hash: 0xbb30a42c1e62f0afda5f0a4e8a562f7a13a24cea00ee81917b86b89e801314aa }) is not equal to the last event RuntimeEvent::Referenda(Event::MetadataSet { index: 1, hash: 0xbb30a42c1e62f0afda5f0a4e8a562f7a13a24cea00ee81917b86b89e801314aa })
  left: RuntimeEvent::Referenda(Event::MetadataSet { index: 1, hash: 0xbb30a42c1e62f0afda5f0a4e8a562f7a13a24cea00ee81917b86b89e801314aa })
 right: RuntimeEvent::Referenda(Event::MetadataSet { index: 0, hash: 0xbb30a42c1e62f0afda5f0a4e8a562f7a13a24cea00ee81917b86b89e801314aa })

With the track caller the error message points to the caller, showing the source of the error:

thread 'tests::set_metadata_works' panicked at substrate/frame/referenda/src/tests.rs:639:9:
assertion `left == right` failed: expected event RuntimeEvent::Referenda(Event::MetadataSet { index: 0, hash: 0xbb30a42c1e62f0afda5f0a4e8a562f7a13a24cea00ee81917b86b89e801314aa }) is not equal to the last event RuntimeEvent::Referenda(Event::MetadataSet { index: 1, hash: 0xbb30a42c1e62f0afda5f0a4e8a562f7a13a24cea00ee81917b86b89e801314aa })
  left: RuntimeEvent::Referenda(Event::MetadataSet { index: 1, hash: 0xbb30a42c1e62f0afda5f0a4e8a562f7a13a24cea00ee81917b86b89e801314aa })
 right: RuntimeEvent::Referenda(Event::MetadataSet { index: 0, hash: 0xbb30a42c1e62f0afda5f0a4e8a562f7a13a24cea00ee81917b86b89e801314aa })

I also improved the error message to include a warning when checking events on block number zero.

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/12760924237
Failed job name: fmt

[bkchr]

Fuck you can read my mind!

I was having exactly the same problem yesterday and wanted to open the same pr today :P