selftests/net: Fix flaky bind_wildcard due to ephemeral port race

bind_wildcard picks an ephemeral port by binding fd[0] with port 0, reads
it back with getsockname(), and then reuses that exact port number for the
other addresses/families in fd[1..7]. This all happens in the default
network namespace, so unrelated processes on the same host are drawing from
the very same ephemeral port range (net.ipv4.ip_local_port_range) at the
same time.

Such a process can independently get assigned that exact port number on a
different, non-conflicting address in the short window between fd[0]'s
bind() and fd[1]'s, making fd[1]'s bind() fail with EADDRINUSE for reasons
that have nothing to do with the wildcard-bind behavior under test, e.g.:

  # bind_wildcard.c:775:plain:Expected ret (-1) == 0 (0)
  # plain: Test terminated by assertion
  not ok N bind_wildcard.v6_local_v4_local.plain

Fix it the same way so_incoming_cpu.c, tcp_port_share.c and
reuseport_dualstack.c already do: unshare into a fresh network namespace
and bring lo up, so there is nobody else to race with for the port.

Signed-off-by: Ricardo B. Marlière (SUSE) <ricardo@marliere.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/bind_wildcard.c

@@ -1,6 +1,8 @@
 // SPDX-License-Identifier: GPL-2.0
 /* Copyright Amazon.com Inc. or its affiliates. */
 
+#define _GNU_SOURCE
+#include <sched.h>
 #include <sys/socket.h>
 #include <netinet/in.h>
 
@@ -716,6 +718,9 @@ static void setup_addr(FIXTURE_DATA(bind_wildcard) *self, int i,
 
 FIXTURE_SETUP(bind_wildcard)
 {
+	ASSERT_EQ(unshare(CLONE_NEWNET), 0);
+	ASSERT_EQ(system("ip link set lo up"), 0);
+
 	setup_addr(self, 0, variant->family[0], variant->addr[0]);
 	setup_addr(self, 1, variant->family[1], variant->addr[1]);