ssh/terminal: run tests only on supported platforms

The tag matches the platforms defined in util*.go
where the most tested logic is defined.

Change-Id: I90f67d988c795738c3effbc8554a933a7cb355d2
Reviewed-on: https://go-review.googlesource.com/112555
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

ssh/terminal/terminal_test.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin dragonfly freebsd linux,!appengine netbsd openbsd windows plan9 solaris
+
 package terminal
 
 import (