This change adds socket system call hooks to existing test cases for
simulating a bit complicated network conditions to help making timeout
and dual IP stack test cases work more properly in followup changes.

Also test cases print debugging information in non-short mode like the
following:

Leaked goroutines:
net.TestWriteTimeout.func2(0xc20802a5a0, 0xc20801d000, 0x1000, 0x1000, 0xc2081d2ae0)
	/go/src/net/timeout_test.go:170 +0x98
created by net.TestWriteTimeout
	/go/src/net/timeout_test.go:173 +0x745
net.runDatagramPacketConnServer(0xc2080730e0, 0x2bd270, 0x3, 0x2c1770, 0xb, 0xc2081d2ba0, 0xc2081d2c00)
	/go/src/net/server_test.go:398 +0x667
created by net.TestTimeoutUDP
	/go/src/net/timeout_test.go:247 +0xc9
	(snip)

Leaked sockets:
3: {Cookie:615726511685632 Err:<nil> SocketErr:0}
5: {Cookie:7934075906097152 Err:<nil> SocketErr:0}

Socket statistical information:
{Family:1 Type:805306370 Protocol:0 Opened:17 Accepted:0 Connected:5 Closed:17}
{Family:2 Type:805306369 Protocol:0 Opened:450 Accepted:234 Connected:279 Closed:636}
{Family:1 Type:805306369 Protocol:0 Opened:11 Accepted:5 Connected:5 Closed:16}
{Family:28 Type:805306369 Protocol:0 Opened:95 Accepted:22 Connected:16 Closed:116}
{Family:2 Type:805306370 Protocol:0 Opened:84 Accepted:0 Connected:34 Closed:83}
{Family:28 Type:805306370 Protocol:0 Opened:52 Accepted:0 Connected:4 Closed:52}

Change-Id: I0e84be59a0699bc31245c78e2249423459b8cdda
Reviewed-on: https://go-review.googlesource.com/6390Reviewed-by: Ian Lance Taylor <iant@golang.org>