[testapp] Re-add testapp to default manifest

This commit re-adds testapp to the default manifest and does some
housecleaning on the module:

- Remove the old serial.* and io.*
- Update it to use current syscall APIs
- Update it to use libj6's higher-level thread API

assets/manifests/default.yaml

@@ -8,5 +8,6 @@ panic:
   - panic.serial
 services:
   - srv.logger
+  - testapp
 drivers:
   - drv.uart