test: support non-host tests

For example, for running tests with -target=wasm or
-target=cortex-m-qemu. It looks at the output to determine whether tests
were successful in the absence of a status code.

src/testing/testing.go

@@ -195,6 +195,8 @@ func (m *M) Run() int {
 	if failures > 0 {
 		fmt.Printf("exit status %d\n", failures)
 		fmt.Println("FAIL")
+	} else {
+		fmt.Println("PASS")
 	}
 	return failures
 }