coregraphics: Add basic test

Author: tmc

macos/coregraphics/coregraphics_test.go

@@ -3,3 +3,10 @@ package coregraphics
 import "testing"
 
 func TestCoreGraphicsValid(t *testing.T) {}
+
+func TestDisplayCreateImage(t *testing.T) {
+	id := MainDisplayID()
+	t.Logf("MainDisplayID: %d", id)
+	img := DisplayCreateImage(id)
+	t.Logf("DisplayCreateImage: %v", img)
+}