Fix org.eclipse.mylyn.team.tests.ChangeSetManagerTest

[ruspl-afed]

Currently we have the following failure:

junit.framework.AssertionFailedError: expected:<1> but was:<0>
	at org.eclipse.mylyn.team.tests.ChangeSetManagerTest.testContentsAfterDecay(ChangeSetManagerTest.java:119)

It seems that ContextActiveChangeSetManager#contextActivated is called without active task and activeChangeSet is not filled.
It may be fixed by activating the task prior to InteractionContext activation, but there is an asserion in the test

		assertNull(TasksUiPlugin.getTaskActivityManager().getActiveTask());

that declares the opposite expectation.