unix: don't defer close raw Socketpair fds in tests

The raw fds are successively wrapped using os.NewFile and will be closed
by (*os.File).Close. Avoids a double close, in the worst case closing an
unrelated fd.

Same as CL 309353 does for package syscall.

Change-Id: I26dffc3fefa90d636cb67a904df0bfc3b131f702
Reviewed-on: https://go-review.googlesource.com/c/sys/+/309689
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_unix_test.go

@@ -206,8 +206,6 @@ func TestPassFD(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Socketpair: %v", err)
 	}
-	defer unix.Close(fds[0])
-	defer unix.Close(fds[1])
 	writeFile := os.NewFile(uintptr(fds[0]), "child-writes")
 	readFile := os.NewFile(uintptr(fds[1]), "parent-reads")
 	defer writeFile.Close()