avr: get go test -target=simavr to work

This patch changes two things:

 1. It changes the default stack size. Without this change, the
    goroutine.go test doesn't pass (apparently there's some memory
    corruption).
 2. It moves the excluded tests so that they are skipped with a regular
    `-target=simavr`, not just when running all tests (without
    `-target`).

targets/simavr.json

@@ -1,5 +1,5 @@
 {
     "inherits": ["atmega1284p"],
     "scheduler": "tasks",
-    "default-stack-size": 512
+    "default-stack-size": 384
 }