ssh/terminal: enable tests for aix

Enable the tests on aix after support was added in CL 151077.

Change-Id: I2dcdaaa54d7c27b7697224e0f3cfab3cf0b52b6a
Reviewed-on: https://go-review.googlesource.com/c/151437
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Clément Chigot <clement.chigot@atos.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: tklauser

terminal/terminal_test.go

@@ -2,7 +2,7 @@
 // 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
+// +build aix darwin dragonfly freebsd linux,!appengine netbsd openbsd windows plan9 solaris
 
 package terminal