Don't run the sock_diag tests twice.

TcpTest inherits from SockDiagTest, which means it runs all the
SockDiagTest test methods as well. Fix this by making the two
inherit from a new base class with no test methods.

Change-Id: I6aa1bb987be00ba29317ab035f2ab3d3cb8e5b9e
diff --git a/net/test/sock_diag_test.py b/net/test/sock_diag_test.py
index 2ca1bb0..eb5eb63 100755
--- a/net/test/sock_diag_test.py
+++ b/net/test/sock_diag_test.py
@@ -37,7 +37,8 @@
 # TODO: Backport SOCK_DESTROY and delete this.
 HAVE_SOCK_DESTROY = net_test.LINUX_VERSION >= (4, 4)
 
-class SockDiagTest(multinetwork_base.MultiNetworkBaseTest):
+
+class SockDiagBaseTest(multinetwork_base.MultiNetworkBaseTest):
 
   @staticmethod
   def _CreateLotsOfSockets():
@@ -51,6 +52,9 @@
       socketpairs[(addr, sport, dport)] = socketpair
     return socketpairs
 
+
+class SockDiagTest(SockDiagBaseTest):
+
   def setUp(self):
     super(SockDiagTest, self).setUp()
     self.sock_diag = sock_diag.SockDiag()
@@ -216,7 +220,7 @@
 
 # TODO: Take a tun fd as input, make this a utility class, and reuse at least
 # in forwarding_test.
-class TcpTest(SockDiagTest):
+class TcpTest(SockDiagBaseTest):
 
   NOT_YET_ACCEPTED = -1