tests: enable test for negative fds in tst-poll

Enable the test for negative fds in poll(). Issue 323 has been fixed.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Jaspal Singh Dhillon <jaspal.iiith@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Revert "build.mk: move modules/tests/usr.manifest to build/"

This reverts commit 7555bca3205d0d55f8d0b46f7f90bbee2481471d.

Jaspal writes:

  This commit breaks 'make check' because the location of
  modules/tests/usr.manifest as changed and module.py needs to be
  updated.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>