[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

src/user/testapp/testapp.module

@@ -5,7 +5,5 @@ module("testapp",
     deps = [ "libc" ],
     description = "Testbed app",
     sources = [
-        "io.cpp",
         "main.cpp",
-        "serial.cpp",
     ])