Add test to check socket get untagged after closed

Add a net test case for xt_qtaguid module to check if a tagged socket is
properly untagged after it is closed. It can be used to verify that the
socket file descriptor is freed from xt_qtaguid module after socket is
closed.

Bug: 36374484
Test: Run qtaguid_test.py in nettest vm and all kernel should pass the
test.

Change-Id: I8f5c0ed0a467165e148829d9bcfc5d7177631076
Signed-off-by: Chenbo Feng <fengc@google.com>