filter.js: Add test coverage for group-pm-with.

Added one little additional test in
people.js to get up to 100% coverage.

frontend_tests/node_tests/filter.js

@@ -328,6 +328,27 @@ function make_sub(name, stream_id) {
         type: 'private',
         display_recipient: [{id: joe.user_id}],
     }));
+
+    predicate = get_predicate([['group-pm-with', 'nobody@example.com']]);
+    assert(!predicate({
+        type: 'private',
+        display_recipient: [{id: joe.user_id}],
+    }));
+
+    predicate = get_predicate([['group-pm-with', 'Joe@example.com']]);
+    assert(predicate({
+        type: 'private',
+        display_recipient: [{id: joe.user_id}, {id: steve.user_id}, {id: me.user_id}],
+    }));
+    assert(!predicate({ // you must be a part of the group pm
+        type: 'private',
+        display_recipient: [{id: joe.user_id}, {id: steve.user_id}],
+    }));
+    assert(!predicate({
+        type: 'private',
+        display_recipient: [{id: steve.user_id}, {id: me.user_id}],
+    }));
+    assert(!predicate({type: 'stream'}));
 }());
 
 (function test_negated_predicates() {

frontend_tests/node_tests/people.js

@@ -572,6 +572,7 @@ initialize();
         assert.equal(msg, 'Empty recipient list in message');
     };
     people.pm_with_user_ids(message);
+    people.group_pm_with_user_ids(message);
 
     var charles = {
         email: 'charles@example.com',