Add tracing support to mdtest (#14935)

## Summary

This PR extends the mdtest configuration with a `log` setting that can
be any of:

* `true`: Enables tracing
* `false`: Disables tracing (default)
* String: An ENV_FILTER similar to `RED_KNOT_LOG`

```toml
log = true
```

Closes https://github.com/astral-sh/ruff/issues/13865

## Test Plan

I changed a test and tried `log=true`, `log=false`, and `log=INFO`

crates/ruff_db/src/testing.rs

@@ -158,7 +158,7 @@ impl LoggingBuilder {
                         .parse()
                         .expect("Hardcoded directive to be valid"),
                 ),
-            hierarchical: true,
+            hierarchical: false,
         }
     }
 
@@ -167,7 +167,7 @@ impl LoggingBuilder {
 
         Some(Self {
             filter,
-            hierarchical: true,
+            hierarchical: false,
         })
     }