ssh: compile multi_auth_test.go on linux only

LD_PRELOAD doesn't work on Windows, and needs tweaking for Darwin.

Change-Id: Ib153b9e63a0a8bcfefa6387f7910292585452c71
Reviewed-on: https://go-review.googlesource.com/92775
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>

ssh/test/multi_auth_test.go

@@ -14,6 +14,8 @@
 // not exist these tests will be skipped. See compile instructions
 // (for linux) in file ./sshd_test_pw.c.
 
+// +build linux
+
 package test
 
 import (