avr: use a stack size of 512 bytes for testing

The goroutine tests are failing with the default 256 byte stack size.

main_test.go

@@ -173,9 +173,7 @@ func TestBuild(t *testing.T) {
 				avrTests = append(avrTests, t)
 			}
 		}
-		opts := optionsFromTarget("atmega1284p", sema)
-		opts.Scheduler = "tasks"
-		runPlatTests(opts, avrTests, t)
+		runPlatTests(optionsFromTarget("simavr", sema), avrTests, t)
 	})
 
 	if runtime.GOOS == "linux" {

targets/simavr.json

@@ -0,0 +1,5 @@
+{
+    "inherits": ["atmega1284p"],
+    "scheduler": "tasks",
+    "default-stack-size": 512
+}