Add assertions to test cases in DesktopTasksControllerTests

Some tests in DesktopTasksControllerTests try to check ops in a
transaction, but don't put it in an assertion.

Bug: 396215361
Flag: EXEMPT test only change
Test: WMShellUnitTests
Change-Id: I329032639aed0e40da91b5d52a47c3b3dc5249ba