Improves test failure messages when matching FlValue arguments.

For example, before:

  Expected arg #2: has setting ("platformBrightness", 0x32f0980)
           Actual: 0x7f81ec005990, 0x32f0900

And after:

  Expected arg #2: has setting ("platformBrightness", dark)
           Actual: 0x27bded0, {textScaleFactor: 0.0, alwaysUse24HourFormat: false, platformBrightness: light}