Improve tests for image output

[jackwilsdon]

Currently we write tests as a massive array of expected pixels;

{
	path: "/jackwilsdon",
	size: 512,
	image: [8][8]color.Color{
		{jwPal.Foreground, jwPal.Background, jwPal.Foreground, jwPal.Background, jwPal.Background, jwPal.Foreground, jwPal.Background, jwPal.Foreground},
		{jwPal.Foreground, jwPal.Foreground, jwPal.Foreground, jwPal.Foreground, jwPal.Foreground, jwPal.Foreground, jwPal.Foreground, jwPal.Foreground},
		{jwPal.Foreground, jwPal.Background, jwPal.Foreground, jwPal.Foreground, jwPal.Foreground, jwPal.Foreground, jwPal.Background, jwPal.Foreground},
		{jwPal.Background, jwPal.Foreground, jwPal.Foreground, jwPal.Foreground, jwPal.Foreground, jwPal.Foreground, jwPal.Foreground, jwPal.Background},
		{jwPal.Background, jwPal.Background, jwPal.Background, jwPal.Background, jwPal.Background, jwPal.Background, jwPal.Background, jwPal.Background},
		{jwPal.Foreground, jwPal.Background, jwPal.Background, jwPal.Background, jwPal.Background, jwPal.Background, jwPal.Background, jwPal.Foreground},
		{jwPal.Foreground, jwPal.Background, jwPal.Foreground, jwPal.Background, jwPal.Background, jwPal.Foreground, jwPal.Background, jwPal.Foreground},
		{jwPal.Background, jwPal.Background, jwPal.Foreground, jwPal.Foreground, jwPal.Foreground, jwPal.Foreground, jwPal.Background, jwPal.Background},
	},
},

The issue with this is that it's a bit unwieldy! It would be nicer to be able to specify test cases like so;

{
	path: "/jackwilsdon",
	size: 512,
	palette: ppic.DefaultPalette,
	image: []string{
		"# #  # #",
		"########",
		"# #### #",
		" ###### ",
		"        ",
		"#      #",
		"# #  # #",
		"  ####  ",
	},
},

Where a # represents the foreground color of the palette and space the background.