Improve debugging by using `#[track_caller]` in system `assert_last_event` and `assert_has_event` (#7142)

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.