Fix for adding panel to panel group via plugin

When adding panels to panel group, it should also be added to the
list of panels at the class atttribute. Otherwise, it will lose the
reference to the panels when _autodiscover() is triggered.

Change-Id: Ifa530d5a39f48d9fd9cd0ea66f6c16594447058b
Closes-Bug: #1329050
Fix order of arguments in assertEqual

Some tests used incorrect order assertEqual(observed, expected).

The correct order expected by testtools is
assertEqual(expected, observed).

Change-Id: Iabf6e55146412b57b6a6544f312f9510f762566a
Partial-Bug: #1259292
Merge "Fix order of arguments in assertEqual"
Merge "Fix for adding panel to panel group via plugin"