MDL-48002 inbound: Update fixtures to add client info

lib/tests/fixtures/messageinbound/SAMPLE

@@ -1,3 +1,4 @@
+----CLIENT----
 ----EXPECTEDPLAIN----
 ----EXPECTEDHTML----
 ----FULLSOURCE----

lib/tests/fixtures/messageinbound/evolution.test

@@ -1,3 +1,9 @@
+----CLIENT----
+OS: Ubuntu 14.04
+Mail Client: Evolution
+Client Version: 31.6.0
+Date: 30/04/2015
+
 ----EXPECTEDPLAIN----
 An ion meets his atom friend on the street and says he's lost an
 electron. "Are you sure?" asks the atom. The ion replies, "I'm

lib/tests/messageinbound_test.php

@@ -110,6 +110,7 @@ class core_messageinbound_testcase extends advanced_testcase {
             'FULLSOURCE'        => true,
             'EXPECTEDPLAIN'     => true,
             'EXPECTEDHTML'      => true,
+            'CLIENT'            => true, // Required but not needed for tests, just for documentation.
         );
         $section = null;
         $data = array();