Quieter fxtest logger

[satazor]

Is your feature request related to a problem? Please describe.

The current fxtest logger provided by fxtest.WithTestLogger is quite verbose, printing everything. When something fails in tests, I have to scroll down all the log messages until I find information about the actual error.

Describe the solution you'd like
An option that would print only errors, akin to how fxevent.SlogLogger where we can do:

	slogger := &fxevent.SlogLogger{Logger: slog.Default()}
	slogger.UseLogLevel(-1000)

To suppress every log, except errors.

Describe alternatives you've considered
Implementing a package like so:

// tbWriter is a io.Writer that writes to the provided test logger.
type tbWriter struct {
	t testing.TB
}

// Write writes the provided bytes to the underlying testing.TB.
func (w tbWriter) Write(bs []byte) (int, error) {
	w.t.Logf("%s", bs)

	return len(bs), nil
}

// QuietConsoleLogger is a fx logger that only logs errors.
type QuietConsoleLogger struct {
	fxevent.ConsoleLogger
}

// LogEvent logs the provided event if it is an error.
func (m QuietConsoleLogger) LogEvent(event fxevent.Event) {
	rv := reflect.Indirect(reflect.ValueOf(event))
	errRv := rv.FieldByName("Err")

	if !errRv.IsValid() {
		return
	}

	err, ok := errRv.Interface().(error)

	if ok && err != nil {
		m.ConsoleLogger.LogEvent(event)
	}
}

// WithQuietTestLogger provides a fx logger for tests that is silent except on errors.
func WithQuietTestLogger(t testing.TB) fxevent.Logger {
	return QuietConsoleLogger{
		ConsoleLogger: fxevent.ConsoleLogger{
			W: tbWriter{t},
		},
	}
}

Is this a breaking change?
No.

Additional context
N/A

[abhinav]

FWIW at my work we've implemented something closer to #1167:
It buffers all the events in memory, and flushes them only if there was an error.
To determine that there was an error, it uses logic similar to what you have above.

Thinking out loud: It might make sense for events to report a severity, which would translate directly to log levels.
Each event could pick its severity based on whether Err is nil or not for its fields.