Don’t print passing unit tests (#43902)

This removes from the output of `./mach test-unit` hundreds of lines
like:

```
        PASS [   0.011s] style_tests str::test_str_join_empty
```

Signed-off-by: Simon Sapin <simon@igalia.com>

.config/nextest.toml

@@ -1,6 +1,7 @@
 [profile.default]
 # Print a slow warning after period, terminate unit-test after 4x period.
 slow-timeout = { period = "5s", terminate-after = 4 }
+status-level = "leak"
 
 [profile.ci]
 fail-fast = false