tests/py: Use libnftables instead of calling nft binary

tests/py: Use libnftables instead of calling nft binary

This adds a simple nftables Python class in py/nftables.py which gives
access to libnftables API via ctypes module.

nft-test.py is extended to make use of the above class instead of
calling nft binary. Since command line formatting had to be touched
anyway, this patch also streamlines things a bit by introducing
__str__ methods to classes Table and Chain and making extensive use of
format strings instead of onerously adding all string parts together.

Since the called commands don't see a shell anymore, all shell meta
character escaping done in testcases is removed.

The visible effects of this change are:

* Four new warnings in ip/flowtable.t due to changing objref IDs (will
  be addressed later in a patch to libnftnl).

* Reported command line in warning and error messages changed slightly
  for obvious reasons.

* Reduction of a full test run's runtime by a factor of four. Status
  diff after running with 'time':

  < 83 test files, 77 files passed, 1724 unit tests, 0 error, 33 warning
  < 87.23user 696.13system 15:11.82elapsed 85%CPU (0avgtext+0avgdata 9604maxresident)k
  < 8inputs+36800outputs (0major+35171235minor)pagefaults 0swaps

  > 83 test files, 77 files passed, 1724 unit tests, 4 error, 33 warning
  > 6.80user 30.18system 3:45.86elapsed 16%CPU (0avgtext+0avgdata 14064maxresident)k
  > 0inputs+35808outputs (0major+2874minor)pagefaults 0swaps

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>