Replace usages of colinodell/testlogger with BufferingLogger from symfony/error-handler, then remove dev dependency on testlogger

In #2903456: Allow kernel tests to fail or expect logged errors (https://git.drupalcode.org/project/drupal/-/merge_requests/14) we created an AssertableLogger that is much more sophisticated than either colinodell/testlogger or Symfony's BufferingLogger. The idea with colinodell/testlogger was that we would now extend AssertableLogger from that.

What we need for #2903456: Allow kernel tests to fail or expect logged errors is so far beyond the capabilities of BufferingLogger (which offers so little code itself) that I doubt the complexity of adding BufferingLogger as a dependency is worth it. Honestly, I thought it was debatable whether colinodell/testlogger was worth it.

Additionally, as I understand it BufferingLogger basically temporarily stores logs and then flushes them on at the end of a request:

    public function __destruct()
    {
        foreach ($this->logs as [$level, $message, $context]) {
            ...
            error_log(sprintf('%s [%s] %s', date(\DateTimeInterface::RFC3339), $level, $message));
        }
    }

I find it hard to forsee the consequences of this behavior change for test DX.