cmd/gogio: don't hang on some e2e test errors

For example, if the test app fails to start on wayland, we'd block
~forever (ten minutes) waiting for it to render its first frame.

We don't have a good solution right now. But at least we can use a
relatively short timeout, to help out the human who rightfully expects
a result within ten seconds.

While at it, remove a sway "get_seats" command, which was a leftover
from my debugging of what input devices are available when running
headless.

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

cmd/gogio/e2e_test.go

@@ -7,6 +7,7 @@ import (
 	"image"
 	"image/color"
 	"testing"
+	"time"
 )
 
 var raceEnabled = false
@@ -148,3 +149,20 @@ func wantColor(t *testing.T, img image.Image, x, y int, want color.Color) {
 			r_, g_, b_, x, y, r, g, b)
 	}
 }
+
+func waitForFrame(t *testing.T, frameNotifs <-chan bool) {
+	// Unfortunately, there isn't a way to select on a test failing, since
+	// testing.T doesn't have anything like a context or a "done" channel.
+	//
+	// We can't let selects block forever, since the default -test.timeout
+	// is ten minutes - far too long for tests that take seconds.
+	//
+	// For now, a static short timeout is better than nothing. 2s is plenty
+	// for our simple test app to render on any device.
+
+	select {
+	case <-frameNotifs:
+	case <-time.After(2 * time.Second):
+		t.Fatalf("timed out waiting for a frame to be ready")
+	}
+}