Quoting commits:

This test shows that `array_map([new TraceableMessageBus(...), 'dispatch'], $messages)` does not work at all,
because `debug_backtrace()` is used in internals to compute the call-site, and it is not considering the fact
that the caller may be:

 * an internal PHP function
 * a callback (this example test scenario)
 * generated code (`eval()` result)

The output of such a failure:

```
1) Symfony\Component\Messenger\Tests\TraceableMessageBusTest::testItTracesExceptionsWhenMessageBusIsFiredFromArrayCallback
Undefined array key "line"

/app/src/Symfony/Component/Messenger/TraceableMessageBus.php:66
/app/src/Symfony/Component/Messenger/TraceableMessageBus.php:36
/app/src/Symfony/Component/Messenger/Tests/TraceableMessageBusTest.php:175
```

To be on the safe side, also removed `compact()` usage, which was making everything
much muddier and harder to comprehend.