Track caller in migration tests

Whenever a migration test fails, it gives the location `tests/harness/mod.rs:141:17`. This location isn't useful, as it's not where the migration test is defined. We should use [`std::panic::Location`](https://doc.rust-lang.org/stable/std/panic/struct.Location.html) and `#[track_caller]` to emit a useful location.

Read tests/harness/mod.rs around line 141 and inspect how migration test failures report their location. Start with std::panic::Location and #[track_caller], then run the migration tests to verify failures point to the migration test definition rather than the harness.

Written by the indexing model from the issue text.