cmd/gogio: rip out cgo deps for x11

Instead rely on more tiny standalone tools. In this case, scrot lets us
take png screenshots, and works well.

On the plus side, we remove some large X Go deps, and we don't need
nearly as much code.

While at it, skip if any of the tools are missing, and actually defer
the cleanup funcs so that they run when we fail the test early.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>

cmd/gogio/e2e_test.go

@@ -39,11 +39,15 @@ type TestDriver interface {
 func runEndToEndTest(t *testing.T, driver TestDriver) {
 	width, height := 800, 600
 	cleanups := driver.Start(t, "testdata/red.go", width, height)
+	for _, cleanup := range cleanups {
+		defer cleanup()
+	}
 
 	// The colors are split in four rectangular sections. Check the corners
 	// of each of the sections. We check the corners left to right, top to
 	// bottom, like when reading left-to-right text.
 	wantColors := func(topLeft, topRight, botLeft, botRight color.RGBA) {
+		t.Helper()
 		img := driver.Screenshot()
 		size := img.Bounds().Size()
 		// We expect to receive a width*height screenshot.
@@ -98,14 +102,10 @@ func runEndToEndTest(t *testing.T, driver TestDriver) {
 	driver.Click(1*(width/4), 1*(height/4))
 	driver.Click(3*(width/4), 3*(height/4))
 	wantColors(red, white, black, red)
-
-	// Run the cleanup funcs from last to first, as if they were defers.
-	for i := len(cleanups) - 1; i >= 0; i-- {
-		cleanups[i]()
-	}
 }
 
 func wantColor(t *testing.T, img image.Image, x, y int, want color.Color) {
+	t.Helper()
 	r, g, b, _ := want.RGBA()
 	got := img.At(x, y)
 	r_, g_, b_, _ := got.RGBA()